Good lens, better focus?

[glass]

Hello,can an expensive good quality lens  get 100% focus where a cheaper lower quality lens, same power  may only get 85% focus on the same telescope? It is just focus I'm thinking of, nothing else. Thanks.

[mcolbert]

it's cat amongst the pigeons time for this one I think, as there may well be many variables others will consider in responding to your question. (about the lens)

I would ask though whether the choice of focuser - the fine control especially needs to be considered as well. This should be an interesting thread.

michael

[Moonshane]

On axis there will be little difference. Off axis, differences will become more obvious depending on the comparables.

[glass]

Thanks for your replies, looks as though an expensive lens by itself would make little difference.

[mikeDnight]

Lenses are precision instruments whether they are telescope lenses or the lenses used in eyepieces. Despite having the correct curvature, a lens that has not been figured accurately will not perform at its optimum. Often you can find this in cheaper refractors, where the out of focus diffraction rings may appear blurry and one side of focus and appear much sharper on the other. This is an indication of spherical aberration. The end result being the telescope will lose definition and contrast. The same applies to poorly figured mirrors.

You asked if a good lens can reach 100% focus, here I'm assuming you mean give a 100% perfect image? Well I suppose the answer would have to be NO, as no lens is ever 100% perfect. However, top end optics are made to extremely high tolerances, far higher than the human eye could detect without specialist equipment. And even if a lens was 100% perfect, it could still not perform as such because of the imperfections in the human eye and in the atmosphere above us. All we can do is try and ensure we get the best performance possible by using the best optics we can afford.

Thankfully most modern scopes and eyepieces are of very high optical quality irrespective of design, the biggest bugbear being our tempremental atmosphere.

Although its difficult to put in percentage terms, aperture for aperture a high quality lens will certainly outperform a lens of lesser quality and can, as far as image quality goes, outperform significantly larger instruments.

Mike

[ronin]

What do you mean by "better focus" ?

The term is odd, a garbage lens will focus light and good lens will focus light, what do you think "good" or "better" focus is?

If an expensive lens made little difference then why do we buy triplet apos over doublet achros in order to get better results?

Just read the section this is in - do you means lens as in the objective lens at the front of a refractor, or an eyepiece?

[glass]

When I say 100 % focus I just mean where it comes to a point and I can say 'it's in focus' ie I'm happy with what I see,As for which lens I'm talking about it is the eyepiece. Thanks for your views all.

[YKSE]

There're a number of reasons for easy achieving focus:

1. faster telescope should be easier than slower ones because of less of depth of view.

2. some eyepieces do snapp into focus better than others, even in my f10 scope.

3. better focuser with smooth, no slope movement will be of help too.

[glass]

Thanks YKSE esp. for point 2. A new focuser would help too, apart from the expence.

[ollypenrice]

There're a number of reasons for easy achieving focus:

1. faster telescope should be easier than slower ones because of less of depth of view.

2. some eyepieces do snapp into focus better than others, even in my f10 scope.

3. better focuser with smooth, no slope movement will be of help too.

Not sure about point 1. The shallow depth of field demands more accurate focus and is less tolerant of slight errors. You're right that in-and-out movements have a faster visual effect but imaging soon tells you that focus is actually easier at slow F ratios.

I agree with Shane that most EPs are OK near the middle but only the premium ones are sharp to the field stop (and often the field is much larger, to boot.) Good EPs will tend to give smaller stars, too, giving the impression of better focus.

I get the feeling that the OP may have a different optical problem?

Olly

[YKSE]

Not sure about point 1. The shallow depth of field demands more accurate focus and is less tolerant of slight errors. You're right that in-and-out movements have a faster visual effect but imaging soon tells you that focus is actually easier at slow F ratios.

I agree with Shane that most EPs are OK near the middle but only the premium ones are sharp to the field stop (and often the field is much larger, to boot.) Good EPs will tend to give smaller stars, too, giving the impression of better focus.

I get the feeling that the OP may have a different optical problem?

Olly

Glad to hear opnions from a great photographer for more accurate answer, I was thinking only from a visual perspective.

I'm though not sure about that most EP are as good on-axis, looking at the diagrams here:

The chromatic aberrations are different on axis for differenct eyepieces, besides, as other aberrations, it grows linearly with the increase FOV, together with distortions. The off-axis aberrations of an eyepiece are easy seen even in my standard C8, but, on-axis difference of eyepieces I have(had) can be seen in my C8 too, after doing some hunting for faint fuzzies. To my limited knowledge of optics, the best optical quality of any eyepiece is on-axis, because there's always off-axis aberrations in all eyepieces, even though aberrations grows with FOV in different rate for different eyepiece design. Using a tracking mount to view objects on-axis is my simple solutions.

[alan potts]

I have spent many hundreds of hours looking at a massive range of targets through some of the best eyepieces on the market, comparing them to shall we say second string models. It is very true central sharpness is difficult to see differences between these two leagues but slight ones are there. However there are other considerations at the centre of the FOV apart from sharpness, light scatter for example, how well does an eyepiece wrestle with the brightness of the planet Venus and to a lesser extent Jupiter.

Most of the faults in these are seen when scopes get faster, sub F5 ish and you view targets towards the edges of field, then other abberations become more obvious and in cheaper eyepieces probably more so.

There is no real subsitute for quality most of the time but everyone has a budget and it is nice when it can stretch to the likes of Pentax, Nikon, Televue and a few others.

Alan