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07-30-2015, 10:16 AM   #1
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Why f:4.5 rather than f:2.8 in "studio zooms"?

This is quite likely a really dumb question, but as a medium format total newb, I have to ask it.

I'm planning the Big Move from K-3 to 645Z. The body is the easy part. Now I'm examining my lens options and this is where it all goes sideways.

My two most used lenses in studio are DA* 16-50/2.8 and DA* 50-135/2.8. And they have exact equivalents in the Full Frame world: 24-70 and 70-200. So if I was planning the move to FF, I'd be ready to order.

But looking over the lens list and in speaking with studio shooters, the two more or less equivalents in the 645 lineup are the 45-85mm and the 80-160mm. Yeah, not quite the same thing but whatever.

However both of these are f:4.5 glass. WTF? Losing a stop and a third of light? How and why is this acceptable to studio shooters? Is there something that happens optically in the 645 system that makes this no big deal? Does the large bright viewfinder somehow make this all better?

<whine> What about my beautiful f:2.8? </whine>


Thanks in advance!

07-30-2015, 10:20 AM   #2
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f/4.5 will give you similar bokeh on medium format as f/2.8 does on full frame.
Besides, it would be huge!

M
07-30-2015, 10:27 AM   #3
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Well, I understand about the DoF and bokeh. This question is less about that -- though it's a minor concern -- it's more about the fact that there will be over a stop less light when I'm focusing on a distant model who is lit only by modeling lamps. This is already an issue for me, especially as I generally resort to manual focus to avoid hunting and slow AF speeds under low light.
07-30-2015, 10:57 AM   #4
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QuoteOriginally posted by bmw Quote
it's more about the fact that there will be over a stop less light when I'm focusing on a distant model who is lit only by modeling lamps
Hence why we always used powerful strobe lights "back in the day"...

M

07-30-2015, 11:06 AM   #5
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f stop is ratio of aperture to focal length, and if the lens is 50% longer (FL) in the larger format, the aperture (diameter) is also 50% larger, As volume and mass are roughly proportional to length times diameter ^2 for same f stop it would be roughly 1.5x1.5x1.5 times the mass, or about 3 times as massive.

And the depth of field is less for the larger format. You need to accept as one goes to larger format the maximum aperture is smaller (smaller diameter to FL ratio). If you are not comfortable with this trade off, then stay with the smaller format. All sorts of trade offs--similarly you will likely rely much more on tripod, etc. It will change your shooting style!
07-30-2015, 11:11 AM   #6
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It's mostly a question of size weight and cost. To get f2.8 at 160mm would mean the largest iris opening would have to be around 57.14 mm (2.25 in). The front element would have to be at least this size. Add to that the iris, barrels and what not and it gets big fast.

At f/4.5 the largest iris opening need only be 35.6mm (1.4 in).
07-30-2015, 11:11 AM   #7
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I'm already relying on the tripod, and I'm budgeting in a sturdier one! And I'm looking forward to the reduced DoF as that's part of the whole MF appeal: it's unique look.

I'm catching the drift of why the lens differences here. Partly historical, partly economics, and partly ergonomics I guess. I can live with that.

Thanks, folks!
07-30-2015, 11:22 AM   #8
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QuoteOriginally posted by bmw Quote
I'm already relying on the tripod, and I'm budgeting in a sturdier one! And I'm looking forward to the reduced DoF as that's part of the whole MF appeal: it's unique look.

I'm catching the drift of why the lens differences here. Partly historical, partly economics, and partly ergonomics I guess. I can live with that.

Thanks, folks!
The 45-85/4.5 and 80-160/4.5 are both already huge lenses. At a constant f/2.8 they would be truly monstrous

07-30-2015, 12:32 PM   #9
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QuoteOriginally posted by bmw Quote
However both of these are f:4.5 glass. WTF? Losing a stop and a third of light? How and why is this acceptable to studio shooters?
Well because they would be huge and really really expensive. And don't you usually stop down for big DoF in studio? How often do you shoot at f 2.8? Studios are the place that are most likely to be well-lit, no?
07-30-2015, 12:37 PM   #10
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My read of what was said is that the concern was over the ability to autofocus or manual focus with the slower lens when wide open - then of course this would be stopped down to whatever the shot required but the open performance limits viewfinder brightness and hence accuracy of the manual focus efforts with modeling lights.
07-30-2015, 12:44 PM   #11
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QuoteOriginally posted by bmw Quote
However both of these are f:4.5 glass. WTF? Losing a stop and a third of light?
Welcome to the world of medium format where f/4.5 is not a slow lens. The reasons fall into two main categories, one having to do with lens design and the other with practical shooting:

1. Fast lenses (e.g. f/3.5 and faster) with large image circles tend to be a little on the huge size and I do mean huge.

2. DOF is a challenge as the frame size increases. Simply put, at commonly-used focal lengths and working distances f/2.8 is not very useful with the 645Z sensor.


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07-30-2015, 12:56 PM   #12
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Well MF sensor > FF sensor, so actually you would get more total light if the lens was f/2.8 on MF than on FF.
Same thing for FF compared with APS-C, as you'd need, what, f/2 constant on APSC in order to get same total light as FF.
Can't do calculations now regarding equivalence between FF and "645 digital" or others.
07-30-2015, 01:28 PM   #13
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QuoteOriginally posted by LensBeginner Quote
Well MF sensor > FF sensor, so actually you would get more total light if the lens was f/2.8 on MF than on FF.
Same thing for FF compared with APS-C, as you'd need, what, f/2 constant on APSC in order to get same total light as FF.
Can't do calculations now regarding equivalence between FF and "645 digital" or others.
Total light? That's meaningless. Take an 8x10 view camera. Place an 8x10 sheet film negative on it - expose. Now take a 4x5 piece and place into the same camera. Expose. Now a 2.25"x2.25" etc. All come out with the same exposure. Sensor size does not change the light per unit area. where it gets tricky is that the size of the pixels may not be consistent and here it is the same as changing the film between shots. Some variation creeps in. This is not due to simply size but practical limitations of sensors that are large vs. small and cost. In truth this mainly manifests in the area of noise - with smaller pixels being less noise tolerant. Look at the performance of the 12mp FF Sony vs. a 30MP Canon in low light to see what I am talking about. However even in this case - the baseline exposure for a given lens in the same lighting with the same ISO settings should remain similar. The extremes come in when you push the ISO capabilities for low light.
07-30-2015, 01:51 PM   #14
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Not sure with the 645Z, but the Pentax 67II has some "bright" focusing screens for slower lenses.

Phil.
07-30-2015, 02:06 PM   #15
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here is a link to a list of "fast medium format lenses" that could perhaps be adapted to your 645: Fastest medium format lenses (Page 1) - Manual Lens: Usage - Manual Focus Forum

M
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