My focal length

[Calzune]

Hi

I just saw a video on some astro gear and he mentioned that he had longer focal length sense he used a crop sensor camera. 

So my question is: I use a Canon eos 1100d (crop factor 1,62?) and use a skywatcher evostar 72ed + field flattner, is my focal length 420 * 1,62 = 680 f? 

 

Here is the link : 

1 min 40 sec

Edited September 23, 2019 by Calzune
Link

[rockinrome]

Focal length is the same, Field of View changes .....

Edited September 24, 2019 by rockinrome
Did no read post correctly - DOH!

[Cornelius Varley]

31 minutes ago, Calzune said:

Hi

I just saw a video on some astro gear and he mentioned that he had longer focal length sense he used a crop sensor camera. 

So my question is: I use a Canon eos 1100d (crop factor 1,62?) and use a skywatcher evostar 72ed + field flattner, is my focal length 420 * 1,62 = 680 f? 

No. The focal length of the telescope is still 420mm. Ignore "crop factor", this does not change the focal length of the telescope. The effective focal length of the telescope + field flattener will be  fl telescope x field flattener magnification.

[Calzune]

Hmm Okey thanks, so I have 420mm then 😄

[Calzune]

2 different answers.. But I trust the one with most posts 😅

[freiform]

You do not change the focal length in terms of 'making things bigger'. You just change your FOV.

 

Sven

Edited September 23, 2019 by freiform

[rockinrome]

19 hours ago, Cornelius Varley said:

No. The focal length of the telescope is still 420mm. Ignore "crop factor", this does not change the focal length of the telescope. The effective focal length of the telescope + field flattener will be  fl telescope x field flattener magnification.

Sorry, this is what I assumed the question was relating too. I know you can't magically increase a scopes' focal length ........

However, the sensor does have a bearing on the image obtained as opposed to normal viewing.

Edited September 24, 2019 by rockinrome

[rockinrome]

17 hours ago, freiform said:

You do not change the focal length in terms of 'making things bigger'. You just change your FOV.

 

Sven

Hi. This is what I *thought* was being asked .....  

[rockinrome]

20 hours ago, rockinrome said:

If field flattener is 1.0x then yes.

MJ

Sorry, I got a little ahead of myself and was thinking about it in terms of what would be obtained as an image as opposed to simply looking at it at the eyepiece.

So, image would *look* bigger from camera because of the 1.6x crop factor.

Hope this now actually helps you rather than confusing the issue.
Please see my later replies to posts above to clarify the answer .........
 

I'll get my coat .........

 

 

Edited September 24, 2019 by rockinrome

[Cornelius Varley]

20 hours ago, Calzune said:

Hi

I just saw a video on some astro gear and he mentioned that he had longer focal length sense he used a crop sensor camera. 

So my question is: I use a Canon eos 1100d (crop factor 1,62?) and use a skywatcher evostar 72ed + field flattner, is my focal length 420 * 1,62 = 680 f? 

 

Here is the link : 

1 min 40 sec

You shouldn't believe everything you watch on youtube.

[davew]

13 minutes ago, Cornelius Varley said:

You shouldn't believe everything you watch on youtube.

Agreed !

However I think this guy should be applauded for having the guts to apologise for his mistake and do it very quickly. How many other YouTubers do that ? 😎

Dave

[ollypenrice]

The term 'crop factor' is an irritating one since it leads precisely to the confusion under discussion! In implying a relationship between the size of the sensor and the focal length it leads people astray. No such relationship exists and, as rockinrome pointed out at the beginning, only the field of view is affected. In a related error, some folks make the mistake of imagining that a small sensor 'zooms in' on an object as compared with a larger sensor. Again this is totally incorrect. Shown on screen at full size (one camera pixel equals one screen pixel) the smaller and larger chip of the same pixel size will give an identical object size. You can obtain finer resolution (more detail) by changing the chip but to do so you must choose a chip with smaller pixels, not a smaller area. (This will only work in practice if the optics, the guiding and the seeing will support this increase in resolution and they may not.)

Calzone's video maker is making both the errors I describe at the beginning. He is plain wrong.

Olly.

Edited September 24, 2019 by ollypenrice

[Louis D]

7 minutes ago, ollypenrice said:

The term 'crop factor' is an irritating one since it leads precisely to the confusion under discussion! In implying a relationship between the size of the sensor and the focal length it leads people astray.

Would you prefer the earlier term 'magnificaton factor'?  That was the going term about 20 years ago when less than 'full size/35mm film' digital sensors started showing up in camera bodies to help people understand why their traditional lens focal lengths weren't providing the image scales they were accustomed to with the new camera bodies.  Enough readers of camera magazines and nascent camera websites argued for the more accurate term 'crop factor' since the image circle was being cropped relative to a traditional 'full size' sensor and there was no optical magnification going on at all.  Depth of field and perspective remained exactly the same as before, just 'cropped' in camera.  Amazingly, there was a wholesale shift away from 'magnification factor' to 'crop factor' within a year or two across the industry.  If you're good with 'magnification factor', feel free to return to it.  As for me, I'll stick with crop factor as being more accurate.

[Cornelius Varley]

14 minutes ago, ollypenrice said:

The term 'crop factor' is an irritating one since it leads precisely to the confusion under discussion! In implying a relationship between the size of the sensor and the focal length it leads people astray. No such relationship exists and, as rockinrome pointed out at the beginning, only the field of view is affected. In a related error, some folks make the mistake of imagining that a small sensor 'zooms in' on an object as compared with a larger sensor. Again this is totally incorrect. Shown on screen at full size (one camera pixel equals one screen pixel) the smaller and larger chip of the same pixel size will give an identical object size. You can obtain finer resolution (more detail) by changing the chip but to do so you must choose a chip with smaller pixels, not a smaller area. (This will only work in practice if the optics, the guiding and the seeing will support this increase in resolution and they may not.)

Calzone's video maker is making both the errors I describe at the beginning. He is plain wrong.

Olly.

He does correct himself and apologises in the comments section after shown the error of his ways.  

[Cornelius Varley]

1 minute ago, Louis D said:

Would you prefer the earlier term 'magnificaton factor'?  That was the going term about 20 years ago when less than 'full size/35mm film' digital sensors started showing up in camera bodies to help people understand why their traditional lens focal lengths weren't providing the image scales they were accustomed to with the new camera bodies.  Enough readers of camera magazines and nascent camera websites argued for the more accurate term 'crop factor' since the image circle was being cropped relative to a traditional 'full size' sensor and there was no optical magnification going on at all.  Depth of field and perspective remained exactly the same as before, just 'cropped' in camera.  Amazingly, there was a wholesale shift away from 'magnification factor' to 'crop factor' within a year or two across the industry.  If you're good with 'magnification factor', feel free to return to it.  As for me, I'll stick with crop factor as being more accurate.

Both terms are inaccurate. Nothing is "cropped" or "magnified". The correct term in astrophotography is image scale. 

[Louis D]

Just now, Cornelius Varley said:

Both terms are inaccurate. Nothing is "cropped" or "magnified". The correct term in astrophotography is image scale. 

But the term originated in terrestrial camera photography and may have been inappropriately applied to astrophotography.  Do you have a problem with the term when used for consumer DSLRs?  Would the term 'image scale' cause less confusion among the unwashed masses when shopping for camera bodies and lenses?  Is it more intuitive?

[Calzune]

Thank you all for all your comments! "👍🏼

[Louis D]

35 minutes ago, ollypenrice said:

Shown on screen at full size (one camera pixel equals one screen pixel) the smaller and larger chip of the same pixel size will give an identical object size.

Yes, but the term 'crop factor' originated for terrestrial cameras where it's the entire frame that is of concern, not just the object being imaged.  The larger chip will show more of the image circle while the smaller chip will show less of it.  Crop factor has been a useful term for nearly 20 years for consumers (especially older ones raised on 35mm photography) to understand what is going on with smaller than 'full size/35mm' sensors.  Truly, the image circle is being 'cropped' relative to a 'full size/35mm' sensor.  Once photographers got used to that, jumping to image scale was more intuitive as they started working with images on their computers.

[freiform]

I think crop factor is perfectly fine. The image appears cropped relative to a full frame sensor. I don't get the confusion. Just leave focal length or magnification out of it and you're all set.

[rockinrome]

1 hour ago, Cornelius Varley said:

He does correct himself and apologises in the comments section after shown the error of his ways.  

<tut> 😪

[ollypenrice]

2 hours ago, Louis D said:

Yes, but the term 'crop factor' originated for terrestrial cameras where it's the entire frame that is of concern, not just the object being imaged.  The larger chip will show more of the image circle while the smaller chip will show less of it.  Crop factor has been a useful term for nearly 20 years for consumers (especially older ones raised on 35mm photography) to understand what is going on with smaller than 'full size/35mm' sensors.  Truly, the image circle is being 'cropped' relative to a 'full size/35mm' sensor.  Once photographers got used to that, jumping to image scale was more intuitive as they started working with images on their computers.

 

2 hours ago, freiform said:

I think crop factor is perfectly fine. The image appears cropped relative to a full frame sensor. I don't get the confusion. Just leave focal length or magnification out of it and you're all set.

Crop factor is fine in terrestrial photography because it allows owners of a particular lens to know what they will get when moving from the old 35mm film format (and full frame digital chips) to APSc.

The term is surely not fine when imported into astrophotography, which is my field, and causes endless confusion such as that seen on the video. The three errors related to this term have all appeared many times before here on SGL. The young man in the video certainly isn't the first. I argue that the term has no place here.  BTW, there are other bad habits lurking in the common usage of terms in terrestrial photography, too. 'Aperture' has become a synonym of 'F ratio' which, again, isn't misleading when the focal length is constant and nobody cares what the aperture of the iris really is. But in buying a telescope we do care what the aperture is and focal reducers do not have the same effect as changes in aperture, so F ratio and exposure time are not related as simply in telescopes as in camera lenses. And then there's the casual habit of using pixel count to mean resolution which, again, is not a disaster if the chip size is a constant (or one of only a few sizes) but in AP the chip size could be anything.

I'll argue for the formally correct use of terms to avoid a quagmire! In this case the terms which really matter to us in given optics are:

Chip size - which determines field of view and must be matched to the corrected circle offered by the optics.

Pixel size - which determines image scale and resolution and is measured in arcseconds per pixel. This must be matched to the resolution of the optics, the local seeing and the guiding accuracy.

Olly

[freiform]

Hi Olly,

I never came across the use of the term for dedicated CCD or CMOS cameras. Only for "terrestrial" devcies, and almost only for APS-C DSLRs, for oen of which this thread was started. And for those devices I stand by my statement. The term crop factor is fine. As it describes exactly what it is. As it is written in this device's specs.

I see your argument, I understand it. And the crop factor is just a relative description of one of the terms you prefer: Sensor size.

And besides that, the confusion regarding the term crop factor has nothing to do with that name! It has to do with people making false, uninformed assumptions and/or spreading (maybe unknowingly!) misinformation that others pick up. Isn't the internet great! I personally have no idea how anybody actually thinking about it could get the idea that a crop factor somehow has any influence on focal length! That is, unless somebody else told him.

Sven

Edited September 24, 2019 by freiform

[davew]

A similar argument could be put forward re the term " Full frame ". Common usage on this and every other Astro forum is wrong as " Full frame " has nothing to do with the size of chip.

The word on the street is that particular ship has sailed and people like me need to get over it 😆

The term crop factor has also sailed so ...

Dave.

 

 

[JTEC]

I’m with Olly on this.  The terminology is loose.  Nothing is being ‘cropped’.  The system projects an image of given size onto the sensor.  Keeping the optics the same and assuming adequate coverage, if you switch to a bigger sensor, the optics will illuminate it and you’ll capture a wider image. If you change the sensor for a smaller one, it’ll only be big enough to receive part of that wider illuminated field and, by comparison with the wide image, it’ll appear ‘cropped’. But ‘cropped’ from what? You’ve just used a smaller sensor that allows a smaller field to be captured for the same focal length. 

I think the crop factor terminology originated, as I think Louis and others have said, when from a base of 35mm camera lens technology we had to get used to smaller sensors being used and it became convenient to pretend that a 50mm lens behaved , in effect, like a longer one - again, it didn’t, it continued to be a 50mm lens but it was now projecting onto a smaller sensor so that a smaller field was recorded - similar to the effect of using a longer lens on the baseline 35mm format that we had been used to working with.

But in astro-imaging with its plethora of focal lengths and sensor sizes, not to mention the complicating effects of flatteners, reducers, etc,  there is no such traditional baseline and it makes no sense to say that an image has been ‘cropped’ as if something has been actively done to chop its dimensions relative to some imaginary standard. We’re dealing with individual outcomes specific to a given system, so perhaps more use, I would argue, to describe this in terms of  focal length and sensor size used.

 

 

 

[ollypenrice]

50 minutes ago, freiform said:

Hi Olly,

I never came across the use of the term for dedicated CCD or CMOS cameras. Only for "terrestrial" devcies, and almost only for APS-C DSLRs, for oen of which this thread was started. And for those devices I stand by my statement. The term crop factor is fine. As it describes exactly what it is. As it is written in this device's specs.

Sven

Hi Sven,,

Sure. I've already agreed with you on 'crop factor' as a term comparing the effects of using lens 'x' on 35mm and APSc chip sizes. But this thread was started precisely because the OP had seen a video in which the presenter was using the term 'crop factor' in a context in which it is meaningless. This didn't begin as a thread about crop factor in camera lenses, it began as a thread about something which doesn't exist, namely 'crop factor' in astronomical telescopes. This was the first sentence of the thread: 'I just saw a video on some astro gear and he mentioned that he had longer focal length sense he used a crop sensor camera.'

My concern was/is to address this point for the OP. I've done so by pointing out why the video is incorrect and it simply is incorrect in this matter. I thought it would be helpful to mention the origins of the term 'crop factor' since it lies at the heart of the confusion. I must admit that I'm surprised by the carelessness with which daytime photographers use terms (notably 'aperture' and 'F stop') but in the context of their work there is no confusion. However, in astrophotography we have to be very careful with terms like aperture, focal ratio, focal length, resolution, image scale and field of view because we are not using them as relative terms but as absolute ones. (So opening up a camera lens by 'one F stop' doubles its clear aperture and so its light grasp. Fine. But, in AP, inserting a 0.5x focal reducer leaves the aperture exactly where it was. No new photons. The incoming object photons land on fewer pixels but this is absolutely not the equivalent of doubling the light grasp.)

Olly