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12-11-2009, 02:56 PM   #1
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Shutter speed vs focal length

I've been doing some reading, have discovered the shutter speed versus the focal length rule (minimum shutter speed = reciprocal of the focal length), have seen a few relevant posts lately in the photo critique forum, and I have a question about it.

For a digital (not full-size format), is it reasonable to assume that one would need to also use the reciprocal of the crop factor as a multiplier if using an older film lens (for instance, if using an older 50mm film lens on a dSLR, the minimum shutter speed would be 1/50 x 1/1.5 = 1/75 sec)? Would that second multiplier be needed if using a made-for-dSLR lens in manual mode?

What sayeth those that know better?

12-11-2009, 03:02 PM   #2
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In my mind the image stabalisation makes up for the cropfactor, so I still stick with the 'old' rule. I haven't really tested this in any scientific way though, but haven't really gotten blurry photos either.
12-11-2009, 03:06 PM   #3
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Interesting thought, though I tend to turn off image stabilization when I use my old lenses and go fully manual. Perhaps an experiment is in order. (Side note: it was actually a few of your recent posts that precipitated my question!)
12-11-2009, 03:11 PM   #4
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QuoteOriginally posted by Judd Quote
For a digital (not full-size format), is it reasonable to assume that one would need to also use the reciprocal of the crop factor as a multiplier if using an older film lens (for instance, if using an older 50mm film lens on a dSLR, the minimum shutter speed would be 1/50 x 1/1.5 = 1/75 sec)?
Yes, to the extent that even the 1/50" was anything more than a rough approximation of a speed that was "likely" to give a person of "average" steadiness "acceptable" results "most" of the time, on prints of a "typical" size viewed at a "comfortable" viewing distance. Did I put enough quotes in there to suggest that this number is really just a rough rule of thumb?

Shooting digital, you might actually tend to want to go with an even *higher* shutter speed, because a picture that looked sharp enough in a 4x6" print from film (all that most people ever did) is unlikely to do so in a 100% view on your computer monitor. On the other hand, as mentioned, SR helps a lot too. Plus, being digital, it doesn't cost me anything to just shoot a bunch and toss the ones that are blurry. So I have no problems regularly shooting at well below 1 / focal length.

QuoteQuote:
Would that second multiplier be needed if using a made-for-dSLR lens in manual mode?
Not sure what you mean here. A 50mm lens is a 50mm lens, period. Doesn't matter is it was made for a DSLR or not, doesn't matter is you use it in manual mode or not. 50 = 50; bank on it. Whether the lens was made for film or digital does not affect a single aspect of the image or the picture taking experience in any way whatsoever.

12-11-2009, 03:12 PM   #5
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QuoteOriginally posted by Judd Quote
Interesting thought, though I tend to turn off image stabilization when I use my old lenses and go fully manual.
Why would you do that??? One of the great things about Pentax is the ability to get SR even with old manual lenses!
12-11-2009, 03:27 PM   #6
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I have heard both sides...

1) the lens has not changed in FL, so use the original FL as your guide

2) the crop factor basically make your lens a longer FL in practice, so use the adjusted FL as a guide

I agree with both... That is: It is true that if you shoot with a 50mm lens, it is still a 50mm lens on any camera. The FOV is determined by the sensor size. Put a 50mm lens on a 2x2 Hassalblad, and it is a wide angle (Yes, the Image Circle has to be larger too). Put that same 50mm on a P/S and the 50mm lens becomes a long telephoto. Because the size of final image recorded is recorded on a much larger or much smaller format....2x2, 1/2.3 and 4/3rds, are all different sizes. Or maybe if you thought it in film sizes.....2x2 vs 8mm subminiture, vs 1/2 frame 35mm

OK, Even though the FL has not changed, the recorded has on a APS-C or APS-H sensor has. So, you can you use an adjusted shutter speed because the image IS MAGNIFIED in Practice. So, just looking through a telescope or a longer FL lens, the image shakes a lot more with the slightest camera movement, because it TOO is magnified as a crop sensor emulates a longer FL.

So, go ahead and use the higher Shutter Speed, But with SR in Pentax Bodies, you can use the old guide too..Except maybe with longer than 100mm lens markings.

For me, if I use between 70-150mm on my telezoom, I will use a minimum of 1/250s. . As I go towards from 150 - 300 (450 FOV) I use 1/500s minimum. But that's just me
12-11-2009, 03:39 PM   #7
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QuoteOriginally posted by Marc Sabatella Quote
Why would you do that??? One of the great things about Pentax is the ability to get SR even with old manual lenses!
This is true. It may seem odd but right now I'm working on improving my skills with the machine and for the moment, I'd like to run it with as many of the electronic nannies turned off as possible. My reasoning is that if I can get a good feel for the interplay of the main variables when operating the camera manually, then I'll be better able to understand and take advantage of the camera's other capabilities later on.
12-11-2009, 04:44 PM   #8
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The "shutter speed = reciprocal of lens focal length" rule has nothing to do with film/sensor size.

It just happens to be co-incident with 35mm format film/camera.

So for that rule, you have to use the 35mm-format-equivalent focal length, not the actual focal length of the lens.

The lens on my little P&S is 5mm FL (28mm equivalent in 35mm format) at the wide end, but I still have to use 1/30 sec as the minimum, not 1/5 sec.

SR is something separate from the original question.

12-11-2009, 05:15 PM   #9
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QuoteOriginally posted by Marc Sabatella Quote
Not sure what you mean here. A 50mm lens is a 50mm lens, period. Doesn't matter is it was made for a DSLR or not, doesn't matter is you use it in manual mode or not. 50 = 50; bank on it. Whether the lens was made for film or digital does not affect a single aspect of the image or the picture taking experience in any way whatsoever.
What I was trying to differentiate is this: As I've read it, conventional wisdom says that an old 50mm (such as my M series 1.7) would act much like a 75mm lens due to the cropping factor. If both multipliers described above hold for this lens, the minimum shutter speed would be 1/75 sec. I also have the 18-55 lens that came with the camera (K100D). If I make the assumption that the 18-55mm lens is matched to the sensor in the camera and I set the focal length to 50mm, my minimum shutter speed would be 1/50 sec. I believe however, that that assumption is incorrect because I just did a quick and simple experiment to determine whether there would be any need for a conversion factor between the two lenses and found that there would appear to be no need. I simply took two photos of the same thing; one with the 50mm (M-1.7) and the other with the 18-55 set at 50mm...and (duh) got the same photograph from both (with better color rendering from the old lady).
Based upon that, I think I should just go with the ~1/focal length regardless of whether I use the old lens or the new one.
Sorry for being such a noob!
12-11-2009, 05:23 PM   #10
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QuoteOriginally posted by Judd Quote
I simply took two photos of the same thing; one with the 50mm (M-1.7) and the other with the 18-55 set at 50mm...and (duh) got the same photograph from both (with better color rendering from the old lady).
1. The "minimal shutter speed = 1/focal length" is a very general guideline, not a rule. Different people have different levels of stability and different techniques of holding the camera. I can take a photo (35mm film format) with a 50mm lens at 1/30 sec with no problem; but my wife needs at least 1/60 sec.

2. Was SR on or off for your test? It has to be off for the test to have any validity.

3. 50mm with an old M lens and 50mm with a current 18-55mm zoom lens are the same thing in terms of focal length. I'm not sure what your test proved.

4. Have you tried taking a photo at 1/5 sec shutter speed (SR off of course) with a P&S camera that has a 5mm FL lens?

Last edited by SOldBear; 12-11-2009 at 05:30 PM.
12-11-2009, 05:39 PM   #11
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QuoteOriginally posted by SOldBear Quote
1
2. Was SR on or off for your test? It has to be off for the test to have any validity.
The point of the test was to determine whether cropping was any different between an old 50mm lens and a new 18-55 set at 50mm, not to determine whether I'd get a blurry photo or not under a given set of exposure conditions. This made the determination for me that the rule should be applied evenly between new and old lenses, which bolsters Marc's argument that a 50mm is a 50mm. It also somewhat explains the 18-55 lens to me. Using the 1.5x rule, it would be considered a 27-82.5 equivalent lens, which would cover roughly 0.5 to 1.5x the range of the 50mm lens that was the kit lens for the old 35mm cameras.
12-11-2009, 05:56 PM   #12
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A quick note: I know that this is just a general guideline like the rule of thirds and a host of others.

I make my living developing analytical chromatography methods in an R&D laboratory, there are all sorts of rules to follow, and in 20 years of doing that I've found that the art is in knowing which rules to break and when to break them. Before one does that though, one needs to know what the rules are, what the limitations of the rules are, and what the potential ramifications of breaking them might be. I'm just learning the rules in this arena, so I may ask some silly sounding questions here and again. Many thanks to all.
12-11-2009, 10:05 PM   #13
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QuoteOriginally posted by Judd Quote
I simply took two photos of the same thing; one with the 50mm (M-1.7) and the other with the 18-55 set at 50mm...and (duh) got the same photograph from both (with better color rendering from the old lady).
Based upon that, I think I should just go with the ~1/focal length regardless of whether I use the old lens or the new one.
You're half right - there is no reason to treat a 50mm lens design for film any differently than a 50mm lens designed for digital. Whatever shutter speed you decide works for one will work for the other. But as explained above, to the extent 1/50" was the "right" shutter speed for 35mm film, 1/75" will be the "right" shutter speed for APS-C digital - again, regardless of which 50mm lens you use. That's because camera shake has *nothing* to do with focal length per se - it's related to angle of view, period. But again, as my previous post tried to make clear, these are just rough approximations anyhow. So test for yourself to see what kind of shutter speeds *you* can get away with.
12-12-2009, 06:01 AM   #14
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Thank to all! I appreciate your comments.
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