Is this true? (Technical question)

[dcclark]

Originally Posted by fletch

No - No - No. If you tried to sell me some of this crap I'd be off like a shot.

ISO is ISO is ISO (Taking into account calibration) just like f/4 is f/4 is f/4

The most likely cause of the different ISOs used in the original question is that the OP and the other photographer ended up with different prictures. Either the ISO 200 shot was darker or the pictures had different compositions so the amount of light was different.

The size of the sensor of the cameras electornics won't affect the ISO you need to shoot at. All of the calibration will be done in camera. Sure a D3 can shoot higher ISOs than a D40 but if you shoot at ISO 200 with both cameras (with the same aperture and shutter speed) you will get the same exposure, it would even be the same if you went and tried a 5 year old Sony P&S camera. ISO 200 would still be ISO 200. End of, it really is that simple.

(Yes calibration may be off in some cameras but that is an unnecessary confusion IMO)

Thanks for catching that, fletch -- I'm slow on the reply. I agree with the above.

It's worth saying that your exposure can change very quickly especially in odd lighting situations. I was shooting at a waterfall recently, with a number of interesting ND filters, and my maximum exposure length would vary from 1 to 5 seconds based just on slight changes in my position. That's because I was working at 200 mm, and the frame really would change that much. So I wouldn't be surprised if the OP and the curiously misguided fellow next to him really just had a similar difference.

[autofocus]

Fletch, I don't believe I ever said that ISO is not ISO, is not ISO. We all know that ISO is a standard, and obviously, ISO 100 in one camera is theoretically the same as ISO 100 in another camera. What adjusting the ISO up or down in your camera does is making the sensor more, or less sensitive to light. Because of the physical constraints of smaller pixel sizes having less light grabbing qualities, the micro in that camera may have to auto adjust the ISO sensitivity up in order to grab enough light to properly expose the image. In that same scenario, a sensor with larger, more sensitive light grabbing pixels may be able shoot the same shot at, let's say 100 ISO instead of 200 ISO, and also properly expose the image at that setting.
What I'm talking about is the difference between one cameras ability to process light vs another cameras ability...in the camera body, the difference is usually based upon the performance of it's electronics.

And Finally...I know that ISO is ISO is ISO

[ttosifa]

I'm not sure I follow any of this (well, except for those saying the OP's original premise was, errr, wrong). But here is an anecdotal story similar to the argument of people saying a lightmeter would be useless if the OP's original statement was valid.

I recently attended a lighting workshop. Multiple Nikon flashes, Pocket Wizards, and the instructor had a D300. About 20 people in attendance. For reasons I cannot fathom, everyone wanted to push the "copy machine button" and have a picture of every setup on their own camera. We just passed around the PWs. I had a D700; others had Canons, Sonys, and Pentaxes, and maybe others. Everybody had to shoot manual because of the PWs. Everybody used exactly the same settings and got exactly the same picture. Nobody even considered that they might need to change the settings to account for their camera model or make!

Changing my ISO speed would have under- or overexposed the picture. Expecting anything different is just silly.

[sfisher]

Originally Posted by auto-focus

In that same scenario, a sensor with larger, more sensitive light grabbing pixels may be able shoot the same shot at, let's say 100 ISO instead of 200 ISO, and also properly expose the image at that setting.

[snip]

And Finally...I know that ISO is ISO is ISO

I'm afraid the first quoted sentence shows that you don't. You are correct when you say that a sensor with larger pixels will gather more light per pixel, but that does NOT affect the ISO of the sensor. It simply means that the camera with the larger pixels will have to amplify the signal less to achieve a given ISO than a camera with smaller pixels.

ISO is basically a mapping of light intensity to final image intensity. It doesn't matter whether you have a tiny pixel and a large amplification or a large pixel and smaller amplification.

[dcclark]

sfisher has it right. I don't know what more we can say -- ISO not something that cameras can "be better at". They can be better at keeping noise down at various ISOs, but ISO is defined as a specific amount of light sensitivity under specific lighting, aperture, and shutter conditions. Cameras are designed so that ISO 100 is exactly the same as on any other camera, so that when you pick up a different camera, you know exactly how it will work. That's why it's called a standard.

[crunch]

Ok here's my take:

Due to quantity and/or quality of glass in a lens, there may be a slight difference in the amount of light reaching the image sensor. This may be more in better vs. worse lens, or may be less(!) HOWEVER, this difference will be minor, in the order of <1/3rd stop.

I did the following test with two of my lenses. The 1st photo was taken with an EF 28-90mm f/4-5.6 III lens (The kit lens from my EOS 300X 35mm camera. Image quality is probably worse than 18-55mm kit lens). The 2nd photo was taken with my 24-105mm f/4 L IS USM lens, which by general consensus is a pretty good lens.

Camera set to manual, zoomed in max to 90mm, metered using the built in meter to f/5.6, 1/60 at ISO100.

Next, change lenses, zoomed to approx same image size using Live view, kept metering settings exactly the same.

Imported into Lightroom, white balance set to "Daylight", exported as 80% sRGB JPG.

Ignoring the obvious quality differences, the image from the better lens is DARKER!!!! In other words, the better lens (the L lens) let LESS light reach the sensor. There goes the OP's buddy's theory

As far as I can tell, the difference is roughly 1/6th stop.

According to my camera's meter, the 2nd photo using the L lens was underexposed (remember I metered using the first lens) by 1/3rd stop. Obviously the camera can't indicate underexposure by less than 1/3rd stop. Secondly you may have noticed I bumped my tripod and the two photos aren't exactly aligned, so that may also affect the meter a bit.

What does everyone make of this?? The only reason I can think of other than the differing amounts of glass, is that due to the quality of the light reaching the light meter being better in the case of the L lens, the camera is able to meter more accurately?

If I have some time I'll upload these to flickr for everyone's perusal.

[crunch]

Just to add some thoughts:

Focal length affects the camera's meter since you are metering from a different part of a scene. However, using manual metering and with a fixed metering, the exposure on a given part of the scene (which may be a fraction at a wide angle and may fill the frame zoomed right in) will remain the same. Aperture is the ratio of the opening with respect to focal length, if the ratio remains constant the exposure remains constant.

Remember in the days of film, an ISO100 film from Kodak or Fuji or Ilford was the same sensitivity (since ISO is a STANDARD) and require the same exposure settings for the same scene. SImilarly, all digital cameras are calibrated to that same standard (some better than others it seems ). If I had repeated my experiment by changing bodies and keeping the lens fixed, the photos would be the same assuming I used ISO100 film and ISO100 on my DSLR. Any minor difference may be due to calibration of the image processor, or the temperature of the sensor, or the developing process of the film. However, the two will be pretty close.

Look at the side of any old manual flash, it gives the aperture value for a given ISO, independent of lens used, and independent of focal length! This wouldn't work if different lenses gave drastically different exposures; similarly focal length does not affect exposure (only metering!).

Last example, the "Sunny 16 rule" says, in bright daylight, shoot at f/16 with a shutter speed of 1/ISO. Again, this is lens-independent! Any given scene has an EV for a given ISO (in the Sunny 16 case, supposing ISO = 125 this is EV15). Any camera, digital or film, is calibrated to give the same set of shutter speeds and aperture values for a given EV at a given ISO (a "set" since EV15 could be 1/125 f/16, or 1/250 f/11, or 1/500 f/8, etc..., all at ISO125)

So as conclusion, the OP's buddy is WRONG. There may be minor differences which DOES NOT mean better lens = more light, and these differences will be MINOR (<1/3rd stop).

Thoughts?

[vandergus]

Thanks for the test Crunch. The results seem reasonable. The exposures are roughly the same but differ a little bit. Nothing to brag about though.

A side note. Did you know that film (as in motion picture) lenses are actually speced as T2.0 (transmittance) instead of f2.0. This is precisely due to the issues you see here. Even though you are using the same aperture between the 2 lenses, the actual transmittance is a little different. Film lenses are speced based on transmittance so shots between cameras can better match exposures, which becomes important when you're cutting from one shot to another.

[crunch]

Just to re-iterate if it got lost in the long posts above: the better lens seems to let LESS light through!

If I have some time I'll try some more scenes with various lighting scenarios, and different lenses too. I have 2 lenses that cover 200 & 300mm, 4 lenses that cover 100mm, 3 that cover 50mm and 2 at 28mm. Should be interesting

[OsmosisStudios]

Originally Posted by crunch

Ignoring the obvious quality differences, the image from the better lens is DARKER!!!! In other words, the better lens (the L lens) let LESS light reach the sensor. There goes the OP's buddy's theory

Not quite. Your L glass has better CONTRAST. Its not that the image taken with it is darker, but it has a better contrast range. It just looks darker.

Therefore, no difference.