DIFF? "Fast" vs "Slow" lens at Small Apertures?

[Frank Petronio]

It can be easier to focus on the ground glass because it will be brighter. However, especially with some older designs, you'll want to double check at the stopped down aperture because some designs will exhibit a focus shift at different apertures.

Generally speaking, amongst the modern lens designs, it's nicer to get the faster wide angles like the 90/4.5 Grandagon in favor over the 90/6.8 because it will be easier to focus (and it also has more coverage but that is another design issue). But speed/brightness comes at the penalty of size, weight, and expense. But they are really nice lenses (no focus shift either)!

One of the frustrating things about the fast normal and portrait lenses is that the shutters they are mounted in do not have very high speeds. You can get 1/1000th (well more like 1/600th) out of a RB or Speed Graphic's focal plane shutter but if you are using the Aero-Ektar at f/2.5 for the short depth of field effect, it's hard not to overexpose outdoors in the daytime. Acme #5 shutters only go to 1/60th, Copal 3s to 1/125th, etc. Even the old Compurs with the Xenotars - hard to get faster than 1/250th in use.

[rdenney]

My reason for asking...if a person never uses a lens "wide open" to its largest aperture... then is there ANY image benefit of a fast lens over a slow lens, when using tiny apertures to maximize depth of field.

If you never use the large aperture, there is no value in having it, other than during focusing on the ground glass. But don't underestimate the value of that.

The issues Frank brings up don't really apply to modern lenses, such as the fast and not-so-fast Grandagon that he mentioned.

Middle-aged lenses also sometimes came in various speeds. Usually, the faster speed was provided only to aid in focusing, and often the instructions suggested stopping down considerably from there to get acceptable performance. Generally, the slower lens if two were available was more optimized for image-quality performance. An example would be the Kodak Ektar (f/4.5) which were really intended as press lenses, versus the Kodak Commercial Ektar (f/6.3) which were intended for use on view cameras, such as in advertising and other work where image quality was more important.

There are other pairs that represent very different designs, such as the Super Angulon f/8 and the Super Angulon f/5.6. The latter has 8 elements and a wider coverage than the 6-element f/8 version. The extra elements gave the designer what was needed to increase the coverage. I suspect those lenses perform equally well when used at the same aperture. I've certainly not noticed a weakness of my 121/8 SA compared to my 90/5.6, except the multicoating on the 90 reduces veiling flare slightly.

Rick "so, yes, slower lenses can be better deals" Denney

[engl]

I have always assumed faster lenses with more elements and bigger front elements to be somewhat more sensitive to flare, but I have not done any comparison of modern multicoated lenses to see if that is really the case.

[Jack Dahlgren]

I have always assumed faster lenses with more elements and bigger front elements to be somewhat more sensitive to flare, but I have not done any comparison of modern multicoated lenses to see if that is really the case.

I'm no expert, but while there is a correlation with more elements and flare (more surfaces) there are other factors (lens geometry) which can have an impact as well. I can imagine there must be cases where a lens with fewer elements can have more flare.

[Roger Krueger]

One of the things I've noticed is that the length of spikes on in-scene lights depends on how far you're stopped down--my 35mm Canon 24/1.4 at f6.3 has serious spikes, my Mamiya Universal 50/6.3 with a similar (21mm equiv.) field of view has zero spikes at f6.3.

I would assume less-directly-measurable stray light issues would also suffer on faster lenses--the bigger front elements gather light whether you're using it or not, and some of it is bound to go astray.

When I tested my Mamiya Universal 250s the 250/8 Tessar was considerably sharper at all apertures than either of their sibling 6/4 250/5's I tested. (btw, any clue what design the 250/5 is? I'd always kind of assumed a Sonnar, given that the other premium lenses in the Uni setup were Biogons and a Planar, but the diagram doesn't look right.)

But the 100/2.8 Planar smokes the 100/3.5 Tessar, especially on CA and below f/5.6.

[Asher Kelman]

One of the things I've noticed is that the length of spikes on in-scene lights depends on how far you're stopped down--my 35mm Canon 24/1.4 at f6.3 has serious spikes, my Mamiya Universal 50/6.3 with a similar (21mm equiv.) field of view has zero spikes at f6.3.

I would assume less-directly-measurable stray light issues would also suffer on faster lenses--the bigger front elements gather light whether you're using it or not, and some of it is bound to go astray.

When I tested my Mamiya Universal 250s the 250/8 Tessar was considerably sharper at all apertures than either of their sibling 6/4 250/5's I tested. (btw, any clue what design the 250/5 is? I'd always kind of assumed a Sonnar, given that the other premium lenses in the Uni setup were Biogons and a Planar, but the diagram doesn't look right.)

But the 100/2.8 Planar smokes the 100/3.5 Tessar, especially on CA and below f/5.6.

What about 8x10 lenses. I can use the PS945, f 4.5 229mm, for full length portraits on my 8x10. I would like to find a similarly fast lens but longer. What's available?

[Ivan J. Eberle]

Tessars in th 135-150mm range can be both sharp and well-corrected by f/22 and fast for easy focusing (f/1:4.5). Just not both at the same time! Plasmats have better all around performance, better coverage, tend to be multicoated and come in modern shutters. At f/22, the 1/15 to 1s shutter timing reliability may prove to be the most significant difference between old and new LF lenses.