A better understanding of lenses and ISO

[deekay]

Hi everyone,
I have been trying to make up my mind about a DSLR. However, the issues of which lens to partner the camera seems to be an important one. For example, the kit lens from Canon for their 350XT is rated at F 3.5-5.6, while the Zuiko lens for the Olympus is rated F 2.8 - 3.5. Just what is the potential effect on the quality of the image and the ISO rating which can be assigned to a particular photo. (I hope I got the numbers right). Canon make an 17-85 lens that costs 3 times as much as the kit lens, and is much faster (lower F stop). Does this mean that the low light performance is even better than the kit lens, or is it just sharper?

If you have a faster lens, such as some of the newer F2.0 lenses from Zuiko, or a better Canon or Nikon lens with F2.8) do you still need to have a high sensitivity for the CCD or CMOS chip that captures the image?

Will the faster lens allow you to take better advantage of the sensitivity of the CCD or CMOS chip?

I may not have worded this very well, but I hope there is enough for people more knowledgeable than me to explain this rather confusing aspect of the new digital camera development.

thanks
deekay

[Trevor Ash]

Hello,

I see a couple hidden questions in your message that I can provide quick answers for. Hopefully it'll help you a little.

Shutter Facts:
1. Shutter controls "How long" the film, ccd, cmos, is exposed to light.

Aperture Facts:
1. Aperture cvontrols "How much" light reaches the film, ccd, cmos, at any given shutter duration.
2. The smaller the f/stop number, the larger the aperture hole, which means more light reaches the film, ccd, cmos.
3. A lens has different characteristics at different apertures. Some of those characteristics being distortion, vulnerability to flare, sharpness, etc. The one we're usually concerned with is depth of field.
4. The larger the aperture (remember larger aperture = smaller f/stop number) the smaller the DOF for the exposure. Lookup "depth of field" to learn more about that if you want.
5. While there are exceptions, it is usually safe to assume that a lens is going to be less sharp at both extremes of aperture values than in the middle of its aperture range.

ISO Facts:
1. ISO is a number describing how sensitive film, CCD, or CMOS is to light.
2. The higher the ISO number, the more sensitive it is to light, which means higher ISO numbers require less light than lower ISO to achieve the same exposure level.
3. Higher ISO's, with their greater sensitivies, also tend to have more apparent noise than lower ISO's. So the more sensitive the film is the noisier the image becomes.

[deekay]

Thanks Trevor

However, it is clear that I was anything but clear in the question. I will have another attempt. Consider two zoom lenses.

Lens 1: rated at F3.5-5.6 - would let in less light than
lens 2: rated at F2.8-3.5 when either at the minimum or the maximum zoom

Therefore, if my understanding it correct (and I am not convinced it is), with Lens 1, you may need to increase the ISO rating in the camera in order to achieve the same level of image brightness as using Lens 2 because Lens 2 is a 'faster lens' and handles low light conditions better than lens 1.

The reason I ask is because many reviewers place a lot of credibility on the ISO level a camera can still produce usable results from. But it seems to me that with a faster lens you may not need higher ISO levels to get acceptable photos without noise.

Is this correct?

Regards
Deekay

Hello,

I see a couple hidden questions in your message that I can provide quick answers for. Hopefully it'll help you a little.

Shutter Facts:
1. Shutter controls "How long" the film, ccd, cmos, is exposed to light.

Aperture Facts:
1. Aperture cvontrols "How much" light reaches the film, ccd, cmos, at any given shutter duration.
2. The smaller the f/stop number, the larger the aperture hole, which means more light reaches the film, ccd, cmos.
3. A lens has different characteristics at different apertures. Some of those characteristics being distortion, vulnerability to flare, sharpness, etc. The one we're usually concerned with is depth of field.
4. The larger the aperture (remember larger aperture = smaller f/stop number) the smaller the DOF for the exposure. Lookup "depth of field" to learn more about that if you want.
5. While there are exceptions, it is usually safe to assume that a lens is going to be less sharp at both extremes of aperture values than in the middle of its aperture range.

ISO Facts:
1. ISO is a number describing how sensitive film, CCD, or CMOS is to light.
2. The higher the ISO number, the more sensitive it is to light, which means higher ISO numbers require less light than lower ISO to achieve the same exposure level.
3. Higher ISO's, with their greater sensitivies, also tend to have more apparent noise than lower ISO's. So the more sensitive the film is the noisier the image becomes.

[Trevor Ash]

Hi Again!

Therefore, if my understanding it correct (and I am not convinced it is), with Lens 1, you may need to increase the ISO rating in the camera in order to achieve the same level of image brightness as using Lens 2 because Lens 2 is a 'faster lens' and handles low light conditions better than lens 1.

This is correct.

Yes, one way to compensate for a lens with a smaller max aperture is to use a higher ISO. I was just trying to point out that while the exposure level would be the same if compensating for a slower lens with a faster ISO, other qualities of the photograph will be different.

But it seems to me that with a faster lens you may not need higher ISO levels to get acceptable photos without noise

This is correct.

Everything in photography is a tradeoff

It sounds like you need to decide whether the tradeoff for saving money by buying a slower lens (but compensating for it by using a faster ISO) is worth the disadvantages. Most of the advantages/disadvantages are in my first post.

[Chunk]

Here's the (not so well informed about this particular gear) recommendation that I would give my kid if he/she posed that question.
Use the kit lens for a while to gain some experience and find out what kind of limitations you find yourself running up against in your normal type of photography.

Each lens (including those made by the camera manufacturer) have their own strengths and weaknesses and can't be evaluated on just one aspect like max aperture. Although that is important, there may be other facters that make a faster lens a poorer choice. Find the individual tests and reviews for the lenses you are considering and use them to help you with your decision.

[deekay]

thanks very much for the answers guys

It seems to me that buying a DSLR camera in what could be considered a transition period in camera history (ie., now) is a difficult proposition. For example, the new 4/3 format may be equivalent to the change from medium format to 35mm decades ago, or it may be a dead end. Who knows what form of direction the DSLRs will actually take?

my needs are for something light and easy to carry travelling that can be set up quickly for panoramic photography, amongst other things. The plan is to print the images on rolls up to A4 width, but longer than A4. My current camera cannot produce images that you can print to this size.

At least I now have a bit more usable information to make an informed decision.

thanks again
deekay

Hi Again!



This is correct.

Yes, one way to compensate for a lens with a smaller max aperture is to use a higher ISO. I was just trying to point out that while the exposure level would be the same if compensating for a slower lens with a faster ISO, other qualities of the photograph will be different.



This is correct.

Everything in photography is a tradeoff

It sounds like you need to decide whether the tradeoff for saving money by buying a slower lens (but compensating for it by using a faster ISO) is worth the disadvantages. Most of the advantages/disadvantages are in my first post.