Newbie question-focus wide open or at shooting aperture?

[noah]

Shooting 10x8, does it matter if I focus with the lens aperture wide open or at the

shooting aperture? It seems logical to me that focussing would be more accurate wide

open, since the DOF will be smaller and the groundglass will be brighter. But I think I

remember hearing that the focus can shift as you change aperture. Is that true?

 

Thanks.

[michael_briggs2]

It depends on the lens that you are using. Generally focusing with the lens wide open is more accurate because of the smaller DOF and brighter image. However, there are some lenses for which the position of best focus changes with aperture. These lenses have imperfectly corrected spherical abberation and so their focal length changes with aperture. Lenses from the past few decades are well corrected for spherical abberation and don't suffer from focus shift, with the exception of soft focus lenses that intentionally have spherical abberation. So if you are using a modern lens other than an Imagon or Fuji-SF, I would focus wide open.

 

Probably the most common older lens with a reputation for focus shift is the Angulon.

 

If you have some lens that you suspect might have focus shift, you could experiment. Focus on a something like a flashlight bulb in a dark room. With the bright bulb, you should be able to focus accurately even with the lens stopped down. Mark the focus position with the lens wide-open, then move the focus off and refocus with the lens stopped down. Is the position significantly and consistently (over several tries) different? Is so, the lens has focus shift.

[dave_moeller]

It all depends on whether the lens you're using shifts focus when you stop it down. This is easy enough to determine...focus accurately with the lens wide open (don't bother with any movements...just focus on something vertical like a phone pole or a fence post), then stop down and see if the image is still in focus. Most lenses don't shift when you stop down, and for those you should focus wide open.

 

By focusing wide open you'll get a brighter image and you'll find it easier to place your plane of sharpest focus where you want it You can then gain depth of field when you stop down.

 

If, by chance, your lens does shift focus when you stop down, then you have to focus at the aperture you'll be shooting at.

 

I can't think of any specific lenses that shift focus when you stop down, but I do know they exist and I always check new lenses just to be sure.

[dave_moeller]

It looks like Michael's a faster typist than I am!

[kurt_remington]

Easy answer focus big, stop down make final adjustments.

[mark_rossano]

Notable examples of commonly-found lens types that have significant focus shift include Angulon, Dagor and (some) Tele-Xenar. There are probably a few others that you'll run into.

[vick_vickery]

Focus shift was a very common problem in years past with convertible lenses when being used with only one element. Be sure to carefully check any old convertibles that you may be using.

[ole_tjugen]

I have to dispute the allegations that Angulons have noticeable focus shift. I have used 90, 120 and 165mm Angulon lenses, and have never seen any sign of focus shift. Now maybe if you use them as convertible (they were originally marketed as such) you will find a focus shift, but not with the whole lens.

 

"Focus shift" is a result of uncorrected spherical aberration, which can make the perceived sharpest focus change as the aperture is stopped sown. To see what it looks like, try putting a simple magnifying glass (or a cheap close-up filter) in a shutter and watch the ground glass as you change the aperture.