IS, shutter speed and sharpness

Hi All,

Please forgive me for what is probably a question that I shouldn't need to ask, but I am going to anyhow!

I have been a DSLR user on and off for a few years now, and my main lens is the Canon 17-85 f/4-5.6 IS. I recently upgraded from a 300D to a 30D, and a good number of the shots I have taken with that particular lens have been somewhat less than sharp when I view them at 100% size. I know the camera itself isn't at fault because shots with my "sharper" lenses (EF-S 60mm macro for example) look fine. This got me to start looking at the better shots I took with my 300D and I realized that almost all of those were taken at higher ISO settings (400 seemed to be the sweet spot). I then realized that most of my 30D pictures were taken at ISO 100 or 200 which more times than not resulted in a shutter speed less than 1/focal length.

Now, this is probably where my mistake has been all this time. I thought that the IS feature of my lens would give me an extra stop or two which meant I could shoot at lower shutter speeds than recommended, yet when I look at my pictures it seems that I only get sharp results when I am either (a) using a tripod or (b) shooting at speeds faster than 1/focal length. I have shaky hands for sure (runs in the family), so I imagine the IS is always making corrections when I am hand holding the camera.

My question is: does the 1/focal length rule of thumb still apply for getting sharp pictures even with an IS lens? I know the IS can help me get low light shots that I wouldn't otherwise, but in general they just aren't that sharp. Maybe I need to just adjust the ISO as I normally would for a non-IS lens.

Any input (or stern corrections) are much appreciated.

There are 2 things to consider.

First is that the rule of 1*focal length is based upon a full frame camera. For a crop camera such as the 30D it is better to use 1.6*focal length to be sure. Although this will vary widely from person to person.

Second is that the advertised ability of "IS" is 4 stops of shutter speed. I think 2 stops is more realistic.

So

100mm x 1.6 = 1/160 second shutter speed without IS.
2 stops of shutter speed would be 1/40.

eddarr wrote in post #8586264
There are 2 things to consider.

First is that the rule of 1*focal length is based upon a full frame camera. For a crop camera such as the 30D it is better to use 1.6*focal length to be sure. Although this will vary widely from person to person.

Second is that the advertised ability of "IS" is 4 stops of shutter speed. I think 2 stops is more realistic.

So

100mm x 1.6 = 1/160 second shutter speed without IS.
2 stops of shutter speed would be 1/40.

Those are good points. I think I am so used to doing the 1.6x math I forgot to put that in the original post.

I had a 17-85, it was my first DSLR lens. Keep in mind that this was my first lens, so if I were to use it today I might get better results. I found that it was no good in low light so I bought the Sigma 24-60 2.8 and fell in love with it right away. Did a few comparo shots and it seemed the siggy was a bit sharper in general. sold the 17-85 and don't miss it. Bought a used 18-55 IS and I think it's a nice little lens!

Bear in mind that 2 stops with IS probably relates to someone who doesn't have shakey hands too. I am sure that IS will allow you to use 2 stops below what you would normally be able to hand hold at, but because you have shakey hands, your starting point would be higher than for someone who doesn't shake.

Also,....IS is great but it won't prevent subject movement. Even if the subject is sitting still he is still breathing and making other small movements. With a live subject I wouldn't drop below 1/100.

eddarr wrote in post #8586264
There are 2 things to consider.

First is that the rule of 1*focal length is based upon a full frame camera. For a crop camera such as the 30D it is better to use 1.6*focal length to be sure. Although this will vary widely from person to person.

This would be true if 1.6 was magnification factor, but it is crop factor.
I do not see any difference between my cropped and FF bodies in terms of min shutter speed.

awo425 wrote in post #8587861
This would be true if 1.6 was magnification factor, but it is crop factor.
I do not see any difference between my cropped and FF bodies in terms of min shutter speed.

It is also an enlargement factor. A 15x22mm image must be enlarged 1.6x more than a 24x36mm image to reach a given final display (print, monitor) size. The dimension of the blur with a given lens and a given shutter speed will be the same regardless of the format, so when the blur gets 1.6x more enlargement from the 15x22mm format, it will show up more at the final display size.

For example: I set up a 5D and a 50D on tripods side by side. I put a 50mm lens on each of them and focus on an old-fashioned audio record turntable with a toy spinning on it. I take a shot with each camera at 1/100.

Each camera records the toy with 0.01mm of blur. It will be the same absolute physical amount of blur on each camera, because the identical 50mm lenses will project the same absolute sized images into each camera and the shutter speed is the same on each camera.

I enlarge each image to 8x10. That requires about a 10x enlargement of the 24x36mm format and about a 16x enlargement of the 15x22mm format.

That means on the final 8x10 print, the blur has been enlarged to 0.1mm on the print from the 24x36mm format, but 0.16mm on the print enlarged from the 15x22mm format. Guess what: A blur of 0.1mm is still too small to see from normal reading distance, but 0.16mm is just large enough to detect as blurred.

That is why you have to increase the shutter speed with the smaller format.

With regard to the 1/focal length thumbrule: Throw it out. It's meaningless. It's like saying, "The average state in the US exports 80K tons of pineapple and has 50K square miles of Arctic tundra," which may be true if you divide the pineapple export of Hawaii and the square miles of Arctic tundra in Alaska by the number of states in the US, but those figures are not true of any particular state.

Your "shake limit" will be personal to you--you can easily test it within a few minutes with a digital camera. Depending on a worthless thumbrule is unnecessary.

When you talk about a "rule of thumb", bear in mind that it only means an approximation. If you want to know how sharp you can get, you have to do a test with a tripod and a cable release with mirror lockup or self-timer with mirror lockup and then some test shots hand-held to see what you can achieve in that way. Shoot at various apertures -- many lenses don't perform so well wide open, and I'd include the 17-85 among them. IS is very valuable, but can't overcome innate obstacles such as camera shake and lens softness.

those last two posts sum it up. personally, I try to forget about the IS when I am using a lens that has it. It is very good for shooting from a moving platform, but I still try to brace as best as I can and use suitable shutter speeds if possible. If I get better shots because I had the IS working for me, great. If not, the same old rules still apply: faster shutterspeed, better technique, wait for the light etc.
I don't think I have ever said " I couldn't have got that shot without my IS"

And lean on stuff whenever possible. Sit, kneel, lean your butt back on a tree, lean your shoulder against a wall, put your elbows on a car hood. Don't depend on only your own two legs unless you really have to.

The terminology used to advertise IS advantages,. ie: 2 to 4 "stops" I feel is dreadfully misleading and a poor choice.

Getting a stop of aperture, ISO or light is not the same as a "stop" of IS... since IS has no effect on shutter speed!

IS is a substitute for a tripod, not for Light, Aperture or ISO.
It can not help with all the things that make s blurred image due to low shutter speed save for your own shaky hands.

They could have said something like "reduces camera shake by 80%."

we were already willing to pony up big bucks for larger apertures, so they equated the IS to stops of light, a commodity they could count on us buying.
Somehow, adding 25% to the price of a lens to add 25% to its ability to capture images in the hands of shakes the clown just doesn't sound as justifiable.
Of all the ingredients of good photography, stability of the camera can be left pretty much up to the operator. Lots of money can be spent on this art. Spend it on stability if you want, but I think that part of it is kind of up to me.

When I had the 17-55 IS for a weekend to use for a wedding, it worked spectuacular. With my Sigma 24-60 f/2.8 I was pretty much limited to 1/60-1/30 depending on the focal length. The 17-55 allowed me to go slightly lower then that, with sharp results. For wedding photographers who have to push their xxD bodies at ISO1600 in dim churches and don't want to use a tripod, it's a must have. Of course the 17-85 IS is really just an outdoor lens so IMO the IS is not that important.

I'd really love to try out the 70-200 f/4L IS to see how low of a SS I can push it. Generally I can go the FL/SS rule.