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07-30-2015, 11:15 AM   #16
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A 50mm is a 50mm is a 50mm, no matter which manufacturer made it (Olympus, Pentax, Nikon, Hassy). The only thing that changes is the FoV.

07-30-2015, 11:41 AM   #17
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QuoteOriginally posted by pepperberry farm Quote
i thought the crop factor was 1.6
QuoteOriginally posted by photolady95 Quote
i always heard it was 1.5
1.52642......
07-30-2015, 01:05 PM   #18
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Look at this page:
Infographic: full-frame vs crop factor lenses | Digital Camera World

Scenario ONE is that you have a FF 50mm lens used on a FF - the image circle fully covers the sensor. (See left hand image)
Scenario TWO is that you use this lens on a cropped sensor or in crop mode on a FF. You get the cropped sensor portion shown. (see left hand image)
Scenario THREE is that you use a lens designed so that the image circle does NOT fully cover the FF sensor on a FF body in non-cropped mode. In this case the portions outside the cropped area will be compromised perhaps heavily vignetted or even black. The central portion in the cropped sensor area will appear just like it did with the FF lens in Scenario 1. (See right hand image)
Scenario FOUR is that you use the cropped lens on a cropped body. In this case the entire cropped sensor would be exposed to the image without issues. It would look identical to SCENARIO TWO. (See right hand image)

Note that they didn't keep the subject exactly in the same way in both images so comparing across them is difficult. But I think it helps to visualize that the image projected is the same - the sensor size either cuts off excess data (using a cropped sensor with a FF lens) or is too big for the image circle (ff sensor with a lens that doesn't cover that sensor) or is just right (ff w/ff; cropped with cropped).

TO make things a little more complex - the lenses labeled as cropped format by Pentax aren't always actually incapable of FF use. There's a lot of effort dedicated to quantifying this in other threads.

The point is the focal length - which affects how much the image is magnified and what the native properties of the lens are - is unchanged. The perspective you get on the sensor is different for the same focal length because of the way the image circle is projected. That same focal length with a lens with a HUGE image circle would be a very wide angle 8x10 lens. The normal FF lens placed on that 8x10 camera would leave a significant unexposed section due to the image circle not matching the sensor (film) size.

Does this help?
07-31-2015, 06:07 AM   #19
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QuoteOriginally posted by bdery Quote
But only the FOV will correlate. The DOF and other imaging parameters will be that of a 50.
wouldn't a 50 1.8 be a brighter overall image on a full frame than if you place the same lens on an aps-c?

07-31-2015, 09:31 AM   #20
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QuoteOriginally posted by Imp Quote
wouldn't a 50 1.8 be a brighter overall image on a full frame than if you place the same lens on an aps-c?
No.

If you take a picture and then crop it, the exposure does not change.

APS-C is just like cropping Full Frame. Nothing more.
07-31-2015, 01:56 PM - 2 Likes   #21
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QuoteOriginally posted by Imp Quote
wouldn't a 50 1.8 be a brighter overall image on a full frame than if you place the same lens on an aps-c?
I'm sending a free 'Equivalence Does Magic' T-shirt your way, Imp! 😛
07-31-2015, 07:22 PM   #22
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QuoteOriginally posted by Imp Quote
wouldn't a 50 1.8 be a brighter overall image on a full frame than if you place the same lens on an aps-c?
If you shine a floodlight on a wall and then take a black bit of cardboard with two holes cut in it - one 24mm across and one 35mm across - are the two spots going to be different in brightness? It is true that the larger circle has more total light on it because it has a larger area - but the light per unit of area is the same and the perceived brightness is the same.
07-31-2015, 07:50 PM   #23
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QuoteOriginally posted by bdery Quote
No. If you take a picture and then crop it, the exposure does not change. APS-C is just like cropping Full Frame. Nothing more.
Sorry, I was unclear. If you take the lens and set it at 2.8 on an aps-c, and then set it to f/4 on a full frame in order to achieve the same DOF, Then you don't have the same amount of light, right?

---------- Post added 07-31-15 at 10:53 PM ----------

QuoteOriginally posted by UncleVanya Quote
If you shine a floodlight on a wall and then take a black bit of cardboard with two holes cut in it - one 24mm across and one 35mm across - are the two spots going to be different in brightness? It is true that the larger circle has more total light on it because it has a larger area - but the light per unit of area is the same and the perceived brightness is the same.
right, that makes sense

07-31-2015, 09:27 PM   #24
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QuoteOriginally posted by Imp Quote
, Then you don't have the same amount of light, right?
Correct, Imp! :-)

Some Total Light fanatics might claim you just up the ISO to compensate, but all they're doing is letting half the amount required fall on a pixel and double the recorded value - along with noise.

This would not be increasing total light at all - its a sham.

Last edited by clackers; 07-31-2015 at 09:41 PM.
07-31-2015, 11:11 PM   #25
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QuoteOriginally posted by clackers Quote
Some Total Light fanatics might claim you just up the ISO to compensate, but all they're doing is letting half the amount required fall on a pixel and double the recorded value - along with noise.
Yes, it is a sham. I have yet to fully understand the logic of "total light". I think there is a basic misunderstanding of how light gathering ability (physical aperture) affects pictorial photography and a misapplication of exposure terminology (stops) to sensor response.

In any case, a discussion of "equivalence" and related obsessions seems to be an eventuality for any thread on focal length or DOF. Why people think there is anything they need to calculate or compensate for is beyond me. Who gives a rip whether the DOF equivalence can be calculated or whether the luminance flux once a correction factor has been applied would be better if you were using a camera other than the one you own?

We are so fortunate to have good tools that makes the above sort of irrelevant.
  • Live view with DOF preview gives a pretty good estimation of DOF and the optical viewfinder only a little less so. If you are not confident, bracket the aperture setting +/- 2 stops or use a DOF calculator.
  • The exposure histogram provides a practical visualization of total light.
    • The first thing to remember is that height of the curve at any point is the number of pixels reporting a particular value. (Histograms are frequency distributions.)
    • The second thing to remember is that values on the left have less data than those on the right.
    • The third thing is that there are hard cut-offs at both the left and the right below or above which the value is clipped at min (black) or max (white)
    • The fourth thing is that bumping the ISO does not increase the sensitivity of the sensor or create more data. In simplistic terms, it adds to the values for pixels that have such and makes up values for those that don't.
So how does this shake out? To be honest, not as clever as one might think. In practice this is how it works for many of us:
  • Even with the ability to preview DOF most of us don't do so. Instead the strategy is be aware when DOF is thin and take care to focus on critical leading edges and detail. The eye (brain) is very forgiving of trailing blur. Make several exposures if needed.
  • If the subject is stationary, a tripod provides the means to make sure we get plenty of "total light". This might sound a little corny. After all, why does our camera have such high shutter speeds if we are going to be spending a lot of time at ISO 100 and longer than 1/30s? I know, it sucks, but to give perspective, much of the work I do with the 4x5 view camera is done on "B" counting off the seconds.
  • If the subject is prone to move, the time for compromise has come meaning that we bump the the sensitivity of the medium (either sensor or film) and hope to deal with the decrease in quality later.
So much for equivalence. What we have is what there is and what it does is easy to determine.


Steve
08-01-2015, 04:45 AM - 1 Like   #26
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QuoteOriginally posted by Imp Quote
Sorry, I was unclear. If you take the lens and set it at 2.8 on an aps-c, and then set it to f/4 on a full frame in order to achieve the same DOF, Then you don't have the same amount of light, right?
It depends on what you mean by "amount of light".

At f/4 you'll have half the light per unit area hitting the sensor of the FF camera compared to f/2.8 on APS-C , you'd need to compensate your exposure settings with a longer shutter or higher iso or by turning up the lights.

However, the FF sensor has twice the area (close enough) so you end up with the same amount of overall light hitting the sensor (assuming the same shutter & lights now), without even using the word "Total". Because of this the following things happen,..



...never mind. Search the forum for "Total Light", grab a big bowl of popcorn, a few beers, plug in the catheter, and have fun...
08-01-2015, 04:55 AM - 4 Likes   #27
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QuoteOriginally posted by BrianR Quote
Search the forum for "Total Light"
You will want to remove your own eyes with a fork afterwards. ☺
08-01-2015, 04:57 AM - 2 Likes   #28
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QuoteOriginally posted by clackers Quote
You will want to remove your own eyes with a fork afterwards. ☺
Good point. Have an eye-sized jar handy.
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