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I need to test the acuracy of a lens

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prinothcat
 
Posts: 662

I need to test the acuracy of a lens

Post Mon Feb 02, 2009 7:59 am


I recently picked up a used Nikkor 300 f/4. I'm pretty sure it's over exposing by close to 2 stops. I know I can just dial in exposure comp until my images look right. Does anyone have a simple way to determine whether or not this lens is behaving as I believe i to be? My thought was to set up a target under controlled lighting and use another lens I trust to check against the 300. Any thoughts are welcome.

dang
 
Posts: 3780

Re: I need to test the acuracy of a lens

Post Mon Feb 02, 2009 1:04 pm


Are you comparing when shooting at infinity, or close focus? Aperture is rated at infinity, but if you shoot macro you'll recall "effective aperture" comes into play as you get closer.
Here's a discussion you might find interesting: http://answers.yahoo.com/question/index ... 822AAqYyPr
:wink:

prinothcat
 
Posts: 662

Re: I need to test the acuracy of a lens

Post Mon Feb 02, 2009 4:38 pm


Shooting regular telephoto stuff. Not macro. I remember now the discussion regarding aperture and macro work. Thanks for the link, it tells me what I intuitively grasp. I still need to determine by how much this lens is off, and decide the most cost effective way to solve the problem. My guess is that the diaphragm isn't closing properly although it closes freely when I move the lever on the mount of the lens. It's only this one lens. The other possibility is that the lens has far more contrast than I am used to....
Chris

dang
 
Posts: 3780

Re: I need to test the acuracy of a lens

Post Mon Feb 02, 2009 8:08 pm


Hi Chris,
Yes, I understand you're not shooting macro with it, I wasn't clear. It's closest focus is around 8 ft. I guess? Effective f/stop could still comes into play at it's closer distances, along with magnification amounts when comparing lenses. If you test against another lens under controlled conditions, I assume you mean inside. That's why I mentioned effective numbers.

Higher contrast shouldn't change aperture by so much, but should give more punch to your unprocessed files. Of course, you could adjust contrast "in camera" and see what happens. Also, try shooting a white wall under sun light and see if there's color halo... this could indicate a lose/misaligned element.

You could test both against a "hand held" meter, my results are usually within 1/3 stop between similar focal lengths.

Lastly, which lens are you comparing, and what type metering are you using? Don't forget angle of view between the two. If angle of view is wider on one, depending on scene, it can cause drastic changes.

I understand you're using Nikon, but I generally find Canon "L" lenses seem faster to me, so this intrigues me. The best I've come up with... quality elements reduce stray light bouncing around, making better use of whats collected. Perhaps this comes into play with your new lens?

prinothcat
 
Posts: 662

Re: I need to test the acuracy of a lens

Post Mon Feb 02, 2009 10:35 pm


dang wrote:Hi Chris,
Yes, I understand you're not shooting macro with it, I wasn't clear. It's closest focus is around 8 ft. I guess? Effective f/stop could still comes into play at it's closer distances, along with magnification amounts when comparing lenses. If you test against another lens under controlled conditions, I assume you mean inside. That's why I mentioned effective numbers.


This is what I had in mind. So far I have no empirical info, just hat perception that htis lens is blowing out highlights, and whites, in direct light. I've been shooting it bracketed and seem to be choosing images from the "under exposed" end of the range.

Higher contrast shouldn't change aperture by so much, but should give more punch to your unprocessed files. Of course, you could adjust contrast "in camera" and see what happens. Also, try shooting a white wall under sun light and see if there's color halo... this could indicate a lose/misaligned element.

take a look at this image,

It's early am light from behind the camera left. It's already -1 1/3. Seems blown out to me. I did no post on this beyond a scale and unsharp mask. Real life was no where near this bright. I do wish I had had an 82mm CP filter handy for this shoot.

You could test both against a "hand held" meter, my results are usually within 1/3 stop between similar focal lengths.

This is where I'm headed with this. If my hand held and the in camera agree, but he image is blown out I think I'll have an idea that the lens needs work. Or I just make sure I dial in the proper EC each time I use it.... For now. Unfortunately I don't have another lens of this caliber to test against. My longest pro caliber zoom is 200mm. The 70-300 prosumer I have is no match for this one in terms of optics, although they are both Nikon.

Lastly, which lens are you comparing, and what type metering are you using? Don't forget angle of view between the two. If angle of view is wider on one, depending on scene, it can cause drastic changes.


I can only objectively compare images to the afore mentioned 80-200 AF-D, and it seems much brighter. Both have similar angles of view.

I understand you're using Nikon, but I generally find Canon "L" lenses seem faster to me, so this intrigues me. The best I've come up with... quality elements reduce stray light bouncing around, making better use of whats collected. Perhaps this comes into play with your new lens?

Could well be. It's the same generation optics as the 80-200 so I think the elements and coatings are similar. Hmm the 300 may not have ED glass in it, perhaps this makes a difference....

dang
 
Posts: 3780

Re: I need to test the acuracy of a lens

Post Mon Feb 02, 2009 11:29 pm


Hi Chris,
A Circular Polarizer would help a lot, I'm sure. It might be the difference in angle the photos are taken from. This shot from the same day looks much better:


As you said, it's also taken with compensation, though the light from behind may be reflecting directly back. This is a difficult situation to shoot, and probably easier to meter on cloudy days. What you might try is using burst mode, and see what happens. With the waves constantly changing, so is the exposure.

Rather than a lens problem, it might be lag between metering and exposing? If "auto exposure lock" engaged when first pressing the shutter half way, it might be a problem if you track before shooting. I'm not familiar with how Nikon works, but might be something to look into.

Interesting, I'll be glad to hear when you figure it out.

prinothcat
 
Posts: 662

Re: I need to test the acuracy of a lens

Post Tue Feb 03, 2009 2:44 am


Dang,
I would agree with you on this. The photo you linked was from a different angle, sun not directly behind but still in the same general direction. It's more what I was expecting. I will pursue the idea about tracking, as I do not remember if I shot these in constant focus mode or AF lock. They were shot in burst mode, bracketed and I picked the best to fool with. Bad habit to some folks I know, but I'm a sports shooter, so it also a hard habit to break. I have some other stuff I've shot with this lens, but it was in horrendous light conditions, back lit, snow, lots of midday glare so I don't think they are fair tests. I guess I really need to sit down and ring this lens out fully. I shot some stuff indoors this afternoon, off a tripod in indirect light and they seem to be ok. More fun and games to ensue.


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