Aperture Variation in Different Lenses?

[brent79]

Hi there,

I'm pretty new to LF and this might be a dumb question...

I recently bought a Rodenstock 210mm 5.6 Apo-Sironar-S lens and have been astounded by the quality compared to the digital slr Ive been used to for the past few yrs. I was using my dlsr to meter for my 4x5 exposures and everything has worked out fine.

I recently got my hands on a Fujinon-W 180mm 5.6 for real cheap. I was just playing around with it and noticed the aperture dial goes beyond f64 as well as in the other direction, it goes beyond f5.6

I went back and had a closer look at the Rodenstock, it stops right on f5.6 but does go a bit beyond f64

I wanted to do a comparison so I set the Rodenstock to f16 and tried to match the aperture on the Fujinon by eyeballing it. The Fujinon reads halfway between f13 and f14.

Is this normal since they are different focal lengths or is there something fishy going on? I'm leaving in 2 days to go on a trip and was planning on bringing both lenses so wont have time to do a test. I will be using my dslr to meter. I'm just concerned if I use the Fujinon I might be overexposing (which shouldnt really matter with film right?) and lacking depth of field

Any ideas would be much appreciated
Thanks in advance!

[JRFrench]

The aperture is a fraction of the focal length. f16 means the diameter of the aperture is 1/16th * the focal length.

So 11.25mm for the 180mm, 13.125mm for the 210mm.

http://en.wikipedia.org/wiki/F-number

[brent79]

Thanks
Learn something new everyday

[Arne Croell]

And it is not the physical diameter of the aperture that is used to calculate the f-stop - it is the diameter of the entrance pupil, which is the apparent diameter of the aperture seen through the front lens. Since this depends on the power of the front lens, the same f-stop might be a different physical diameter for different lenses even with the same focal length.

[Jack Dahlgren]

Your shutter speeds are likely to vary much more than any variance from listed aperture. If you got a lens for very cheap - you might want to check the shutter speed before you go. There have been some posts here about checking shutter speed by using your dslr. The basic process: put your lens over the dslr and point at an evenly lit featureless subject. set the shutter speed on dslr to desired speed and open the lens and take a shot using dslr shutter. Then set your shutter speed on the lens. Set your shutter speed on the dslr to t or whatever will keep it open long enough. Take a shot using the lens shutter. Compare the two resulting images. Do this across the range of shutter speeds. You can then compare the two lenses you have and the dslr shutter and see where there is inconsistency. No need to fix it if it is consistent but slow (or fast) as long as you know this and compensate.

Film has some latitude so you don't need to be too exacting. Other items like subject distance etc. come into play when making the actual exposure, so just check to see if there are no major problems.

[Brian Ellis]

And to further confuse things, lenses often aren't the exact focal length they say, the stated focal length may just be a rounded number. So a 105mm lens may really be 102mm or a 108mm or anything in the near vicinity of 105mm.

[Ken Lee]

"I recently bought a Rodenstock 210mm 5.6 Apo-Sironar-S lens and have been astounded by the quality compared to the digital slr Ive been used to for the past few yrs. "

Yes, that's one of the best lenses out there for Large Format.

I presume you're using it on 4x5. Give it a try on 5x7 (for which it is a "normal" length), and you may be astounded even further.

[Steve Hamley]

LF shutters are a standard item and are not designed with specific lenses in mind. So when lenses of different focal lengths and maximum apertures are fitted to a "standard" shutter, there can be some movement of the aperture lever "left over" at either end (or both ends) of the travel.

Cheers, Steve