Lens Testing Tutorial

[twade]

To all,

I have published a lens testing tutorial for astrophotographers.  It includes four lenses that I have recently tested.  Although, the camera used was a Canon 5D Mark II, the article relates to film as well.  It's just more time consuming to test film.  Please, let me know if you find any errors.  I'm not an optics expert so some of the listed aberrations may be in error.  In addition, if you see any aberrations not listed, please let me know and describe them so I can mention them in the article.

http://tinyurl.com/ydq3pee

Enjoy,

Wade

P.S.

Don't forget to check out the 85mm f/1.2 bokeh link.  It includes several images I've taken with this awesome lens over the past several weeks.  One of the images is of a very old pug named Hercules.  He turned 14 years old yesterday (9/30).

[Simon Hicks]

Hi Wade,

I can't get through to the link. I've tried before with the tinyurl thingy to try and see some of your images, but I just get an error message. Am I doing something wrong? 

Cheers
         Simon

[twade]

Simon,

Try this link:

http://www.northwest-landscapes.com/html/articles/lens-testing.html

Perhaps, your software firewall or browser is blocking the expansion of the tinyurl link.

Wade

[Simon Hicks]

Hi Wade,

Thanks for the new link....that worked just fine. I love the mouse over vs F#.....that's the best illustration of it that I have seen. It really shows the need to back off to about F4 to get good results.

It would be interesting to see how a 50mm F4 lens at F4 performs against a 50mm F1.4 lens at F4. I presume the later lens would perfom much better....but interesting to see the comparison.

Cheers
          Simon

[Jordi Gallego]

Very good lens tutorial Wade, thanks for posting!

[twade]

Simon and Jordi,

Thanks!

Simon

I love the mouse over vs F#.....that's the best illustration of it that I have seen.

I'm glad you like it.  I was torn between it and Flash.  I didn't want to require Flash so I went with mouse-overs.

It really shows the need to back off to about F4 to get good results.

I was hoping for better on the F/1.4 lenses.  I can't wait to use the 24mm on aurora since I should be able to use F/1.6 or so.

It would be interesting to see how a 50mm F4 lens at F4 performs against a 50mm F1.4 lens at F4. I presume the later lens would perfom much better....but interesting to see the comparison.

It would be interesting indeed.  I'm actually a little surprised it takes to F/4.0 to produce decent stars with the Sigma.  It's supposed to be a pretty good lens.  Perhaps, I got a bad copy.  In fact, all the lenses may be bad copies.    There's no telling since I can't afford to buy more than one copy to compare them.  Perhaps, someone out there can do a comparison with there's.

Wade

[Simon Hicks]

Hi Wade,

I spoke to some of the guys at my astro society about your tests (and how illustrative the mouse over method was). One of them stated that consumer camera lenses....i.e. ones designed to cover a huge range of applications....usually in daylight....are designed for their optimum performance at about F4. I don't know whether this is true...I'm no expert....but it does sound plausable.

Which them made me think....you can get telescopes by the hundred that come with focal lengths of say 300mm to 2000mm. These are designed to be fast and flat.....i.e. you don't ever close down an iris on a Meade SCT to get a higher F# (well vary rarely....maybe for solar work). And telescope designers go a long way to try and get pin point stars right across the FOV....obviously some better than others. And they don't have 7 to 23 pieces of glass in the optical train which is what camera lenses have.

So its clear that a 500mm focal length telescope will usually outperform a 500mm focal length camera lens on astrophotography applications....simply because they are optimised in different ways.

So this prompts the question....why are there no telescope-like lenses in the range 18mm to say 200mm?

I would be very surprised if I had just managed to be the first person in history to discover this untapped niche in the market  .....so I'm obviously missing something. Is it just that commercial (i.e. mass produced and cheap) lenses are 'good enough'?

Cheers
         Simon

[georg.viehoever]

Simon,

I am not an expert in optics, but I guess the difficult bit is to build lenses with low F-ratios. Building an 18mm f lens with an aperture (light gathering opening) of 18mm (which is small by telescope standards) gives you an F-ratio of 1. I guess it is not possible to build lenses or mirrors that give you an F of 1 or below (at least I have never seen one).

http://www.telescope-optics.net/index.htm#TABLE_OF_CONTENTS has some very nice material on the basics of telescope optics. Maybe you find a more profound answer there...

Georg

[Simon Hicks]

Hi Georg,

Yup....an 18mm focal length lens with say a 50mm diameter aperture would have an F# of 0.36. So this would obviously have massive aberations - to the extent of being unusable.

So I guess a standard camera 18mm F1.8 lens, which has an approx 50mm diameter piece of glass at the front, maybe has a multistage compression arrangement which brings the rays in more gradually....which results in more lenses, slower F# and less aberations.

Maybe I should go on a lens designing course for beginners...I'm just guessing at the moment! 

Cheers
         Simon