Is aperture affected by crop factor!!! 10d

Lenses have their f-stops programmed in memory, and it makes sense that the dslr’s use these f-stops as an arbitrary guide to keeping things simple. However I keep pondering this silly idea since these lenses are been used in cameras with 1.6 and 1.3x crop factors. Which leads me to ask a stupid question!!! As with my 10D I would think a 1.6x crop and the idea that the light being used is the light only from the center most part of the lens. Which in turn would be the part that pulls the most light in the first place. So I am asking that since it’s a 1.6x crop wouldn’t the light be affected as well. I would think that 24 mm lens being cropped at 38.4 mm in a 10d would make the f/stop of a 2.8 lens more closer resembling a 1.8 or 2.0. If this is true wouldn’t the iso equivalence that is we love so much truly be more in the range of 50-800 iso’s with a boost of 1600 iso’s instead of the reported 100-1600, and 3200

Please correct me if I am wrong I need to know so that I can stop debating with myself over this one.

Fstop has absolutely nothing to do with crop factor. Tstop is influenced a tiny amount, but not worth mentioning.

That's it, the whole story.

Jason

dewayne howse said:

Lenses have their f-stops programmed in memory, and it makes sense
that the dslr’s use these f-stops as an arbitrary guide to keeping
things simple. However I keep pondering this silly idea since
these lenses are been used in cameras with 1.6 and 1.3x crop
factors. Which leads me to ask a stupid question!!! As with my 10D
I would think a 1.6x crop and the idea that the light being used is
the light only from the center most part of the lens. Which in turn
would be the part that pulls the most light in the first place. So
I am asking that since it’s a 1.6x crop wouldn’t the light be
affected as well. I would think that 24 mm lens being cropped at
38.4 mm in a 10d would make the f/stop of a 2.8 lens more closer
resembling a 1.8 or 2.0. If this is true wouldn’t the iso
equivalence that is we love so much truly be more in the range of
50-800 iso’s with a boost of 1600 iso’s instead of the reported
100-1600, and 3200

Please correct me if I am wrong I need to know so that I can stop
debating with myself over this one.

Click to expand...

dewayne howse said:

Lenses have their f-stops programmed in memory, and it makes sense
that the dslr’s use these f-stops as an arbitrary guide to keeping
things simple. However I keep pondering this silly idea since
these lenses are been used in cameras with 1.6 and 1.3x crop
factors. Which leads me to ask a stupid question!!! As with my 10D
I would think a 1.6x crop and the idea that the light being used is
the light only from the center most part of the lens. Which in turn
would be the part that pulls the most light in the first place. So
I am asking that since it’s a 1.6x crop wouldn’t the light be
affected as well. I would think that 24 mm lens being cropped at
38.4 mm in a 10d would make the f/stop of a 2.8 lens more closer
resembling a 1.8 or 2.0. If this is true wouldn’t the iso
equivalence that is we love so much truly be more in the range of
50-800 iso’s with a boost of 1600 iso’s instead of the reported
100-1600, and 3200

Please correct me if I am wrong I need to know so that I can stop
debating with myself over this one.

Click to expand...

F-stops assume that lenses have no light fall-off. With lenses that do have it, the corners are underexposed: the center receives the "correct" amount of light that you'd expect from the aperture. So the crop factor does mitigate light fall-off, as the darker corners are cropped out, but not the aperture value itself. An f/2.8 lens is f/2.8, no matter how you crop it.

Petteri
--
Portfolio: [ http://www.seittipaja.fi/index/ ]
Pontification: [ http://www.seittipaja.fi/ ]

dewayne howse said:

Lenses have their f-stops programmed in memory, and it makes sense
that the dslr’s use these f-stops as an arbitrary guide to keeping
things simple.

Click to expand...

F-stops aren't arbitrary. They refer to actual aperture diaphragm diameters. Obviously they vary with focal length...f/2.8 on a 24mm lens refers to a smaller aperture opening than f/2.8 on a 200mm lens.

dewayne howse said:

However I keep pondering this silly idea since hese lenses are
been used in cameras with 1.6 and 1.3x crop
factors.

Click to expand...

"Crop factor" is a semi-bogus term introduced so people can apply 35mm film measurements to other formats (the 10D being a camera that uses a different film/sensor format). IMO this causes confusion at least as much as it helps people understand the behavior of a camera like the 10D.

dewayne howse said:

As with my 10D I would think a 1.6x crop and the idea that the light
being used is the light only from the center most part of the lens.
Which in turn would be the part that pulls the most light in the first
place. So I am asking that since it’s a 1.6x crop wouldn’t the light be
affected as well.

Click to expand...

No, the amount of light falling on the 10D's sensor is pretty much the same as the amount falling on a 35mm film frame or the sensor in the 1Ds. With some lenses there is some light falloff in the corners at wider apertures, and this is less of an issue with the 10D than with a "full-frame" camera. But it's not that significant.

dewayne howse said:

I would think that 24 mm lens being cropped at 38.4 mm in a 10d...

Click to expand...

Just to note...it's not the lens that's being "cropped" with the 10D. The sensor is smaller but this has no effect on lenses. "24mm" doesn't refer to any particular field-of-view so on the 10D it doesn't become a "38.4mm" lens. It's still a 24mm lens but it projects its image onto a smaller sensor so the resulting image has a narrower FOV than you'd get with the same lens on a 35mm film camera or 1Ds.

-Dave-

I had wondered about the same thing, until I reasoned out why it isn't true.

The amount of light entering the lens is controlled by the aperture size, or f-stop. But the light level for exposing the sensor (or film) is based on the amount of light per unit area. So ot doesn't matter how big or small the film or sensor is.

Think about it another way: Imagine taking a "correct" exposure with a 35mm camera using film, developing it, and then cropping the developed slide or negative down to the 1.6 crop factor with a pair of scissors. Wouldn't you expect the cropped film to still be correctly exposed or not?

dewayne howse said:

Lenses have their f-stops programmed in memory, and it makes sense
that the dslr’s use these f-stops as an arbitrary guide to keeping
things simple. However I keep pondering this silly idea since
these lenses are been used in cameras with 1.6 and 1.3x crop
factors. Which leads me to ask a stupid question!!! As with my 10D
I would think a 1.6x crop and the idea that the light being used is
the light only from the center most part of the lens. Which in turn
would be the part that pulls the most light in the first place. So
I am asking that since it’s a 1.6x crop wouldn’t the light be
affected as well. I would think that 24 mm lens being cropped at
38.4 mm in a 10d would make the f/stop of a 2.8 lens more closer
resembling a 1.8 or 2.0. If this is true wouldn’t the iso
equivalence that is we love so much truly be more in the range of
50-800 iso’s with a boost of 1600 iso’s instead of the reported
100-1600, and 3200

Please correct me if I am wrong I need to know so that I can stop
debating with myself over this one.

Click to expand...

Thinking back some 55-60 years when I first read just what an f-stop was, it is quite simply f=focal length, and the hole size is f/x, thus it is a given fraction of the focal length.

Assume a 50mm lens...assume it is set at f4. 50mm/4 = 12.5mm, or 1/4 the focal length. The hole letting in light is 12.5mm in diameter, or 1/2 inch.

Assume a 200mm lens at f4...200mm/4 = 50mm or about 2 inches.

So, an f-stop simply states the size of the apeture through which light is passed. Period.

bob snow

I think that the non-DSLR digital cameras use a marketing slight of hand.... Here are the statistics from the Minolta Dimage 7i (my previous camera).

The sensor is 8.8mm wide -- giving a 4.0x crop.
The zoom range is 7.2 - 50.8 (which means 28mm to 200mm with the crop)
The f-stop is advertised as 2.8 - 3.5.

(Here is DP's picture of the front of the lens with the information clearly marked:



)

OK, let's do some math: A 35mm camera with a 200mm lens would need 200mm / 3.5 = 57mm front glass collecting light. A zoom with a 7x range would probably need quite a bit bigger piece of glass than this, since some of the aperature is wasted by the wide-range zoom. However, the Dimage 7i takes 49mm filters! Clearly, the front glass is less than 57mm.

So, it seems to me that point-and-shoot cameras calculate the focal ranges using the crop factor but then "conveniently" use non-cropped focal when computing the f-stop. You can't have it both ways!

Accordingly, to compare all cameras, we should probably convert EVERYTHING to 35mm equivalents. Thus when you compute f-stop, you should use the crop-adjusted focal length to do the calculation (or use the non-cropped focal and multiply the resulting f-stop by the crop).

Put another way, compare the Canon 1DS to the 10D using the exact same lens. The same number of photons enter the lens for a given exposure, but the 10D throws away 1 / 1.6 of those photons! Thus for purposes of comparing light-gathering, the 10D would be operating at 1.6x the f-stop.

Peter K Wagner said:

I think that the non-DSLR digital cameras use a marketing slight of
hand.... Here are the statistics from the Minolta Dimage 7i (my
previous camera).

The sensor is 8.8mm wide -- giving a 4.0x crop.
The zoom range is 7.2 - 50.8 (which means 28mm to 200mm with the crop)
The f-stop is advertised as 2.8 - 3.5.

Click to expand...

I don't think there is any sleight of hand here. It's a 7.2mm to 50.8mm zoom, with an effective f-stop range of f/2.8 to f/3.5.

Peter K Wagner said:

OK, let's do some math: A 35mm camera with a 200mm lens would need
200mm / 3.5 = 57mm front glass collecting light. A zoom with a 7x
range would probably need quite a bit bigger piece of glass than
this, since some of the aperature is wasted by the wide-range zoom.
However, the Dimage 7i takes 49mm filters! Clearly, the front
glass is less than 57mm.

Click to expand...

Just a quick note -- the aperture is 57mm, not the front element. The front element can be much larger, but likely will be at least 57mm large. You probably knew this already, but just to clarify ;-).

That's a 50.8mm lens on the long end, so the physical size of the aperture wide open is effectively 14.5mm, not 57mm. I say "effectively" because optics designers use tricks sometimes to make the effective aperture not the same as the physical aperture -- that's how a wide open lens is f/2.8 for the same physical aperture size whether you are zoomed out at 200mm or in at 70mm with the 70-200/2.8.

In any case, use the actual focal length of the lens to determine aperture.

Peter K Wagner said:

So, it seems to me that point-and-shoot cameras calculate the focal
ranges using the crop factor but then "conveniently" use
non-cropped focal when computing the f-stop. You can't have it
both ways!

Click to expand...

I understand your frustration, but I don't think they are cheating. The lens says it is a 7.2mm to 50.8mm zoom, it says it is f/2.8 to f/3.5 variable aperture. That's exactly what it is. The fact that the "effective focal length" refers only to the field of view and doesn't affect aperture is not sleight of hand. Heck, DPR is filled with messages talking about how DOF is not the same for an equivalent field of view with a 1.6x DSLR versus an SLR -- and now you understand why!

Peter K Wagner said:

Accordingly, to compare all cameras, we should probably convert
EVERYTHING to 35mm equivalents. Thus when you compute f-stop, you
should use the crop-adjusted focal length to do the calculation (or
use the non-cropped focal and multiply the resulting f-stop by the
crop).

Click to expand...

No offense, but this makes no sense. Some people (including me!) shoot with formats larger than 35mm and I hate it when everything is stated in terms of 35mm as if that's all there is ;-). The bottom line is the focal length is the focal length, the f-stop is the f-stop, and that's all there is to it. Keep in mind the focal length isn't some number pulled out of thin air, it's the bellows distance from the nodal point of the lens to the film/sensor when focused at infinity. If your film or sensor is tiny, you only capture a small image and a tight field of view. If your film or sensor is large, you capture a large image and a much wider field of view.

Peter K Wagner said:

Put another way, compare the Canon 1DS to the 10D using the exact
same lens. The same number of photons enter the lens for a given
exposure, but the 10D throws away 1 / 1.6 of those photons! Thus
for purposes of comparing light-gathering, the 10D would be
operating at 1.6x the f-stop.

Click to expand...

Not really, but again, I understand your frustration when it comes to tiny sensor P&S digicams ;-).

--
Brian Kennedy
http://www.briankennedy.net/

Peter K Wagner said:

The zoom range is 7.2 - 50.8 (which means 28mm to 200mm with the crop)

Click to expand...

I'm not sure I understand your point. The lens is really a 7.2-50.8 mm lens. The aperture would be measured with respect to that real focal length, not with respect to the 35mm equivalent.

The 35mm equivalent numbers just tell you what the angle of view would be in 35mm terms, under the assumption that you're familiar with what angle of view a 28mm lens provides on a 35mm camera.
-harry

Peter K Wagner said:

I think that the non-DSLR digital cameras use a marketing slight of
hand.... Here are the statistics from the Minolta Dimage 7i (my
previous camera).

Click to expand...

Every manufacturer VERY clearly states their focal length and aperture on the front of the lens. I've never seen an equivalent anything on the front of a lens.

Peter K Wagner said:

Put another way, compare the Canon 1DS to the 10D using the exact
same lens. The same number of photons enter the lens for a given
exposure, but the 10D throws away 1 / 1.6 of those photons! Thus
for purposes of comparing light-gathering, the 10D would be
operating at 1.6x the f-stop.

Click to expand...

But the reality is there is no such thing as an equivalent F-stop, and this would also throw off everything like light meters, guide numbers, etc. If we did this, we'd have to throw out every Fstop on 35mm lenses too since those are really just cropped large format cameras. This all stems from the idea of a "focal length equivalent" which doesn't exist and so an equivalent f-stop doesn't exist. If you want to compare effective light gathering or

DOF capabilities, you need to compare sensor sizes as another distinct measurement.

Jason

I somewhat thought that out the same reasoning you did... but I got to looking at an 85mm 1.8 and 1.2 and I got to thinking about the difference in the size of the glass and thought the subject was worth asking.

R. HREBIC said:

I had wondered about the same thing, until I reasoned out why it
isn't true.

The amount of light entering the lens is controlled by the aperture
size, or f-stop. But the light level for exposing the sensor (or
film) is based on the amount of light per unit area. So ot doesn't
matter how big or small the film or sensor is.

Think about it another way: Imagine taking a "correct" exposure
with a 35mm camera using film, developing it, and then cropping the
developed slide or negative down to the 1.6 crop factor with a pair
of scissors. Wouldn't you expect the cropped film to still be
correctly exposed or not?

Click to expand...

I really like the way you explained it. I makes perfect sense, Us youngens need more teachers like you!!! You get down to the nitty gritty and gives us the facts!!!

Bob Snow said:

Thinking back some 55-60 years when I first read just what an
f-stop was, it is quite simply f=focal length, and the hole size
is f/x, thus it is a given fraction of the focal length.

Assume a 50mm lens...assume it is set at f4. 50mm/4 = 12.5mm, or
1/4 the focal length. The hole letting in light is 12.5mm in
diameter, or 1/2 inch.

Assume a 200mm lens at f4...200mm/4 = 50mm or about 2 inches.

So, an f-stop simply states the size of the apeture through which
light is passed. Period.

bob snow

Click to expand...

hm said:

I'm not sure I understand your point. The lens is really a
7.2-50.8 mm lens. The aperture would be measured with respect to
that real focal length, not with respect to the 35mm equivalent.
The 35mm equivalent numbers just tell you what the angle of view
would be in 35mm terms, under the assumption that you're familiar
with what angle of view a 28mm lens provides on a 35mm camera.
-harry

Click to expand...

Fortunately, the front of the lens does indeed state 7.2mm-50.8mm f2.8 - f3.5. That's just fine.

But look at PD's own summary page for the Dimage 7i: http://www.dpreview.com/reviews/specs/Minolta/minolta_dimage7i.asp

See where it says zoom wide= 28, zoom tele = 200, aperture f2.8 - f3.5 ? Those figures seem to be picking and choosing when to apply the crop factor and when not to apply the crop.

Peter K Wagner said:

Fortunately, the front of the lens does indeed state 7.2mm-50.8mm
f2.8 - f3.5. That's just fine.

But look at PD's own summary page for the Dimage 7i:
http://www.dpreview.com/reviews/specs/Minolta/minolta_dimage7i.asp

See where it says zoom wide= 28, zoom tele = 200, aperture f2.8 -
f3.5 ? Those figures seem to be picking and choosing when to apply
the crop factor and when not to apply the crop.

Click to expand...

This is the kind of misleading info that ends up in print (or on the Web) due to the "crop factor" and "effective focal length" nonsense. There is no real "crop factor" in play here nor is there a real "effective focal length." These are terms of convenience (or, perhaps more accurately, laziness) intended to be used for rough comparison that have unfortunately been widely adopted as gospel truth. The D7i's lens is not a 28–200mm zoom and should never be referred to as such. Minolta states the real truth: 7.2–50.8mm.

We'd be much better off in terms of understanding, I think, if compact digicam makers provided field-of-view stats for their lenses. If Minolta were to stencil "7.2–50.8mm (75–12º diagonal)" or something similar on the D7i, and if people were educated to think in terms of degrees of coverage rather than focal lengths (which don't equate to any particular degrees of coverage), most of the misunderstanding would disappear. You'd look at the 75–12º figure and understand, without needing to know the particular focal lengths involved, that the lens zooms from medium wide angle coverage to medium telephoto.

-Dave-