Why step the f down on a lens?

[autonm]

I have the usual 1100d canon camera with a nifty fifty lens.

Which allows a f 1.8

I have read several times of stepping the f down.

Q. Why? ... How does this improve the image or what difference will I see?

Q. Should I just stip it down to 2.0 or even further?

Many thanks.

[steppenwolf]

This does improve the image in a couple of ways. Firstly it increases the depth of field so the effects of field curvature will be lessened so stars will be better shaped. Secondly, the effects of chromatic aberration are reduced.

The downside - which is worth putting up with - is that the leaves of the iris will cause diffraction artefacts (spikes) and exposure lengths will increase.

A 1.8 would benefit from stopping down to f2.8 or f3.5

Sent from my iPhone from somewhere dark .....

[kirkster501]

I was wondering this too.  Thanks for the explanation.

[autonm]

Steve... Thanks for the reply.... Funny thing is I'm actually reading through your book right up this second

Hoping for a lottery win. As currently I'm a dob mount and a dslr ... No tracking

So it's quite appropriate that u replied.

[knobby]

Not sure if this is accurate but by stopping down a tad I've always felt that it takes away the extreme edges of the lens which are the hardest bits to get right in design

[ollypenrice]

Not sure if this is accurate but by stopping down a tad I've always felt that it takes away the extreme edges of the lens which are the hardest bits to get right in design

That is what lies behind Steve's explanation. Fast optics are hard to make and, therefore expensive.

Astronomers are always working at infinity. However, daytime photographers aren't and may want a very fast lens simply because it has a shallow depth of field. They may actually want this in order to control what parts of an image are in focus and what aren't. (Eg on a portrait you normally want the face, and most of all the eyes, in focus but the walls of the room or the background scenery need to be way out of focus so as not to draw attention.

Longer FL lenses can be stopped down with a home made aperture mask on the front. I know Steve has done this, as have I. I suspect it might not work on short FL lenses though. It gets rid of the diff effects from the iris when it works.

Olly

[Digz]

Daytime photographers may also want fast lenses because they let more light in allowing faster shutter speeds to freeze action, for example for sports or wildlife photography without the expense of upping ISO and increasing noise......

[kirkster501]

My master plan is to have the main imaging rig going (scope and CCD) with the modded 450D and lens on the top doing a wide field under control of a remote control, programable shutter release.  I can then get "free" wide field pictures dirven and guided.   Does that make sense?

[JB80]

Lately I have been using the nifty fifty at f4.0, the results seem good enough so until otherwise directed I think I'll stick to it.

[steppenwolf]

My master plan is to have the main imaging rig going (scope and CCD) with the modded 450D and lens on the top .... I can then get "free" wide field pictures dirven and guided. Does that make sense?

Makes perfect sense although the clunk of the DSLR mirror *may* unsettle things a little but I doubt it,

Sent from my iPhone from somewhere dark .....

[RikM]

Lately I have been using the nifty fifty at f4.0, the results seem good enough so until otherwise directed I think I'll stick to it.

I run mine at f/4 as well.

[kirkster501]

Makes perfect sense although the clunk of the DSLR mirror *may* unsettle things a little but I doubt it,

Sent from my iPhone from somewhere dark .....

Well, nothing like trial and error.. . I shall report back after we next get a clear night. In 20 years at this rate......

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[ollypenrice]

My master plan is to have the main imaging rig going (scope and CCD) with the modded 450D and lens on the top doing a wide field under control of a remote control, programable shutter release.  I can then get "free" wide field pictures dirven and guided.   Does that make sense?

Perfect sense.

Olly