Relative ISO vs Aperture Light Comparisons?

[waltgary]

The above title is not very accurate but I'm not sure how to improve it.


I'm getting the 5d II and am struggling between the 24-70/2.8 vs 24-105/4 IS. I've owned the 24-70 but needed IS, I like my 17-55/2.8and hesitate to go to f/4. I belive that the reduced noise of the 5d II (vs my 40 and 50d) may compensate for that?


In other words, is there a relative rule of thumb when comparing ISO to aperture settings? I see in the reviews on this site something to the effect that going from 2.8-4 lets in twice as much light? At what ISO change would this be equal so to speak? Low light sports are what I'm thinking, with or without a flash.


I realize these are somewhat stupid questions, but I really need IS (wobbly) and low light capability. If simply going from say, ISO 100 to 200 makes for the same relative exposure, I should be good with the f/4. The $390 price difference is significant too.


I suspect the obvious answer is to mount my 70-200/2.8 ISon the 5d and keep the 50d/17-55 EF-S. That's an appealling choice but I wouldn't mind lugging only one body around rather than two.


I'd like to thank all who take the time to read this uneducated and somewhat rambling post. Thedigitalpicture.com has been very helpful to me through the last few years.


gary

[Mark Elberson]

Someone please correct me if I am wrong but a stop is a stop is a stop. What I mean is the difference between f/2.8 and f/4 is a stop of light. The difference between ISO 100 and ISO 200 is a stop of light. The difference between 1/60 and 1/30 is a stop of light. If you change any one of those variables you will need to change one of the other ones to keep the correct exposure.


For your specific question regarding the 24-70 f/2.8 or the 24-105 f/4 you would need to shoot at ISO 200 with the f/4 as opposed to ISO 100 with the f/2.8 assuming the same shutter speed. Make sense?

[Jon Ruyle]

Not a stupid question.


My take is that since the 5d II sensor is 2.5 times as big as the 50d sensor, you should be able to use an f/4 lens on the 5d II with at least as fast shutter speeds as f/2.8 on the 50d (with similar nosie).


That is, iso 1000 on the 5d II should have the same signal noise ratio (so similar noise) as iso 400 on the 50d. If you use the f/4 lens on the 5D II at iso 1000, you'll have a slightly faster shutter speed than using an f/2.8 lens on the 50d.


Put another way, the 2.5 times as much light hitting the sensor more than compensates for the 1/2 as much light per unit area of the f/2.8 lens. (So the answer to your question, if I understand it, is yes).


I'm not claiming that all of this is actually true in real life, but that would be true other things being equal. It is probably pretty close to being true between 5d II and 50d.

[peety3]

A stop is a stop is a stop, at least with regards to satisfying the meter. If you had a 24-70/2.8 aimed at a still subject at f/2.8 and ISO 100, then replaced it with the 24-105/4IS and changed to f/4 and ISO to 200, you'd have the same shutter speed. You wouldn't have the same image, as elements of the photo might come into sharper focus because of the larger depth-of-field at f/4. With the same shutter speed, you've have the same subject motion blur, but would have no/less camera shake blur thanks to the IS if your shutter speed is between 8/focal-length and 1/focal-length (theoretically no blur) or slower than 8/focal-length (less blur).

[peety3]

In simpler terms, every time you open up/close down the aperture by a factor of 1.4, that's one stop. 4*1.4=5.6, so f/4 to f/5.6 is one stop less. 4/1.4 = 2.8, so f/4 to f/2.8 is one stop more. Every time you halve/double the shutter speed, that's one stop. Every time you halve/double the ISO, that's one stop.


If you're doing low-light sports, aperture is almost always key, and IS rarely saves the day. Why? You need fast shutter speeds for stopping the action, often 1/250th or better. Hand-held, most people can hold a camera down to shutter speeds of 1/focal-length (so 1/200th for your 70-200). Therefore your need for 1/250th shutter speed means your picture will natively be faster than the typical limits of hand-holdability. IS matters in the range of N/focal-length to 1/focal-length, where N is 4 for early IS units capable of 2 stops of effect, 8 for middle-vintage IS units capable of 3 stops of effect, or 16 for the latest lenses with units capable of 4 stops of effect (i.e. 2 to the X power where X is the number of stops of correction), as mentioned in my previous post.

[Jon Ruyle]

I agee that a stop is a stop, no argument there []


My point was only that you shouldn't expect more noise with a f/2.8 lens on a 1.6fovcf camera vs a f/4 lens on a full frame camera, even if you crank up the iso on the ff to get the same shutter speed... the bigger sensor will- other things being equal- more than compensate for that difference.


As for depth of field, you'll actually get less dof at f/4 with the full frame camera as compared to f/2.8 on the with 1.6fovcf camera... assuming the shot is framed the same.


But your point is taken. There are a lot of other factors that will make the picture different- for better or worse.

[peety3]

Fair points, I was basing my comments on the OP's debate between the 24-70 and 24-105 for his pending 5D2 purchase.

[Daniel Browning]

I'm getting the 5d II and am struggling between the 24-70/2.8 vs 24-105/4 IS. I've owned the 24-70 but needed IS, I like my 17-55/2.8 and hesitate to go to f/4. I belive that the reduced noise of the 5d II (vs my 40 and 50d) may compensate for that?

Yes, it will completely compensate for it. The 24-105 f/4 IS has a wider aperture and more light gathering power than the 17-55 f/2.8 at every equivalent field of view and perspective. The reason for this goes back to the true definition of aperture, which is focal length divided by f-number. F-number is not aperture, though the two concepts are often confused.

I posted about this topic here:
http://community.the-digital-picture.../42/94.aspx#94

For example, the aperture on the 17mm at f/2.8 is 6mm. The L at 27mm f/4 is 7mm: that's wider aperture. The larger front element of the L points toward this fact too.

One important difference is that f/4 does not activate the f/2.8-only autofocus sensors.

The focal ratio (not aperture) is one stop narrower (f/4 vs f/2.8), so the light intensity per area is twice as dim. However, the total amount of light is more than double, thanks to the much larger area and wider aperture. So you can up the ISO or use -1 EC to get the same shutter speed as with f/2.8 and still collect a half-stop more light in total.

At f/2.8 you will be letting in twice the amount of light *per area*, but the total amount of light captured will be much smaller. For example, the lens on a very tiny 1/1.7" Digicam with a 7mm f/2.8 lens also has the same perspective/FOV as your 20mm, and it also captures the same amount of light per area, but the total amount of light falling on the sensor is much less. In fact, f/2.8 on a digicam is the same amount of light as f/14 on your 50D! (And f/19 on the 5D2.) It's no wonder digicams struggle in low light. The reason is sensor area.

In the same way, a full-frame f/4 lens focuses more light than an f/2.8 APS-C lens: it's just spread out over a wider area. So the full-frame camera has about a 1/3 stop advantage in low light at f/4: you could upgrade to the 5D2 and 24-105 f/4 and still get all the same DOF, noise level, and shutter speed.

But the amount of light project by the lens is only half the equation: the other part is the technology of the sensor. The 5D2 is using older sensor technology from the 1DS3, but it has an improved ADC (less read noise) and more permissive color filters (slightly higher sensitivity/QE). But the 50D has newer sensor technology such as gapless microlenses to add on top of the ADC/CFA changes, so the net result is that that 50D is slightly more efficient per area. I'd guess around 3-6%. So given a lens with the same aperture (not f-number), the 50D would be superior to the 5D2. But as I said above, the 24-105 has a slightly *wider* aperture: 1mm at the wide end. This difference is more than the 3-6% technology difference, so the 5D2 is better in the end.

Of course, since we're only talking about differeneces of 5-10%, it's better to say that they are effectively the same. So you shouldn't think that 24-105 f/4 on the 5D2 will be significantly less noise than 17-55 f/2.8 on 50D. But at least it will be the *same* (i.e. you're not taking a step backwards.) To really *improve* noise would require more aperture, that is, f/2.8 on the 5D2.

There is one web page that explains all of this, and more, in excrutiating detail.

http://www.josephjamesphotography.com/equivalence/

I see in the reviews on this site something to the effect that going from 2.8-4 lets in twice as much light?

Right.

At what ISO change would this be equal so to speak?

One stop would mean double the ISO (f/2.8 ISO 100 would become f/4 ISO 200, just as you stated).

I realize these are somewhat stupid questions

Not at all.

If simply going from say, ISO 100 to 200 makes for the same relative exposure, I should be good with the f/4.

I think you'll love it!

Someone please correct me if I am wrong but a stop is a stop is a stop.

I disagree.

What I mean is the difference between f/2.8 and f/4 is a stop of light.

I would not describe it as a stop of "light", but rather a stop of "light intensity per area". That is an important difference. F-number doesn't describe the total amount of light in the resulting image. If it did, there would be no difference between a tiny f/2.8 lens on my camera phone, digicam, APS-C, 35mm, medium format, or a 10-thousand pound telescope. All those are f/2.8, and they all have the same intensity of light per area, but the difference in area and total light makes for a very different image in the end.

I'm not claiming that all of this is actually true in real life, but that would be true other things being equal.

FWIW, I claim that it's true in real life.

A stop is a stop is a stop, at least with regards to satisfying the meter. If you had a 24-70/2.8 aimed at a still subject at f/2.8 and ISO 100, then replaced it with the 24-105/4IS and changed to f/4 and ISO to 200, you'd have the same shutter speed. You wouldn't have the same image, as elements of the photo might come into sharper focus because of the larger depth-of-field at f/4.

Actually, the depth of field projected by the 24-105 at f/4 is *thinner* than the same field of view projected by the 17-55 at f/2.8. It would require stopping down to f/4.5 in order to get the same DOF. The reason, as above, is physical aperture.

I hope that helps.

[peety3]

A stop is a stop is a stop, at least with regards to satisfying the meter. If you had a 24-70/2.8 aimed at a still subject at f/2.8 and ISO 100, then replaced it with the 24-105/4IS and changed to f/4 and ISO to 200, you'd have the same shutter speed. You wouldn't have the same image, as elements of the photo might come into sharper focus because of the larger depth-of-field at f/4.

Actually, the depth of field projected by the 24-105 at f/4 is *thinner* than the same field of view projected by the 17-55 at f/2.8. It would require stopping down to f/4.5 in order to get the same DOF. The reason, as above, is physical aperture.

Seriously, read the first post. The OP is buying a 5D2, and is debating between the 24-70L and the 24-105L. Let's give the OP some honest, logical, understandable advice. To do so, let's stop focusing on the 17-55 lens he has, and stick to the two lenses he's choosing amongst. Once you do that, I think you'll all have a hard time saying the 24-105/4 lets in more light than a 24-70/2.8. Clearly there's an assumption that the two lenses are being used on the same 5D2 camera that I don't think the OP has purchased yet.

[Daniel Browning]

Seriously, read the first post.

I did. Very carefully. Before I posted.

The OP is buying a 5D2, and is debating between the 24-70L and the 24-105L.

Did you read the first post? He said a lot more than just that. He even considered keeping the 50D with 17-55 because of the I.S., but would prefer not to.

Let's give the OP some honest, logical, understandable advice.

I did.

To do so, let's stop focusing on the 17-55 lens he has, and stick to the two lenses he's choosing amongst.

No, I think it's important to also discuss the lens he has. His principle question was whether the 24-105+5D2 and higher ISO could match the 50D+17-55. It can. So now he can choose between getting similar images to what he has now (24-105) or getting images with less noise but no IS (24-70).

Once you do that, I think you'll all have a hard time saying the 24-105/4 lets in more light than a 24-70/2.8.

I never said that. In fact, I said the opposite.