Canon 70-300 is f4/5.6 usm 60d blurry and soft photos

[Robodot]

I brought this up a few weeks back without any real solid answer so I thought i would provide one to anyone who might be interested in putting a full frame lens on a crop sensor camera like i did.

I independently came to a conclusion by testing further, this lens was only sharpest at 300mm with F9+. I recently found out that many, many, many people who own this lens came to the same exact conclusion. as I did.

So i dug a little more (trying to decide to keep Canon or go to another brand) this is what I discovered:

When you use a full frame lens (EF) on a crop sensor camera, not only do you multiply the crop factor for the focal length, you also must "multiply the aperture" by that crop factor.

So on my lens, instead of being a f4/5.6 as Canon states is really a F6.3 / F9.

what does this mean in the real world, you will not be able to separate the subject from the background or use in lower light levels without noise.

[Waddizzle]

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@Robodot wrote:

I brought this up a few weeks back without any real solid answer so I thought i would provide one to anyone who might be interested in putting a full frame lens on a crop sensor camera like i did.

I independently came to a conclusion by testing further, this lens was only sharpest at 300mm with F9+. I recently found out that many, many, many people who own this lens came to the same exact conclusion. as I did.

So i dug a little more (trying to decide to keep Canon or go to another brand) this is what I discovered:

When you use a full frame lens (EF) on a crop sensor camera, not only do you multiply the crop factor for the focal length, you also must "multiply the aperture" by that crop factor.

So on my lens, instead of being a f4/5.6 as Canon states is really a F6.3 / F9.

what does this mean in the real world, you will not be able to separate the subject from the background or use in lower light levels without noise.

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Uh, nope.  I do not think so.  

There is a very simple test to debunk that theory.  Put the lens on a full frame camera, adjust for proper exposure, and take a photo.  Now, put the lens on a APS-C body, dial in the same exposure, and take a photo.  Guess what happens?  Of course, both shots will be properly exposed.  If the aperture crop factor existed, the photo taken on the APS-C body should be underexposed.  But, it will not be.  

This rumor arises from a simple misunderstanding of physics.  Suppose light is falling on a full frame sensor with a given intensity.  Now suppose that same light intensity is falling on an APS-C sensor.  Guess what happens?  Of course, the smaller sensor will receive fewer photons than the full frame sensor, and the ratio of the difference will be the same as the ratio of their total surface areas, which just so happens to be the crop factor.

[Waddizzle]

BTW, many consumer priced telephoto lenses are not their sharpest at minimum aperture.  

I have a Sigma 150-600mm “C”, which is a little soft at minimum aperture, but razor sharp at f/8.  But, I perform AFMA adjustments on it at minimum aperture because the DOF becomes too wide at narrower apertures like f/8 to make the test meaningful.

[ebiggs1]

"When you use a full frame lens (EF) on a crop sensor camera, not only do you multiply the crop factor for the focal length, you also must "multiply the aperture" by that crop factor."

Total nonsense!  Where do you people come up with this stuff? 

 Totally agree with, my friend, waddrizzle on this one.

"Uh, nope.  I do not think so.  

There is a very simple test to debunk that theory..."

However there is even an easier way to see it.  Place a sheet of letter size paper on the table, shine a light on a 6 1/2" circle drawn on that paper. Now draw a 4" inch (1.6 crop) circle on it. SHine the the light again.  Each circle was equally exposed. The light that falls outside of either circle is simply wasted. It effects neither exposure.

It always comes back to the fact that a lens is a lens is a lens and it is always the same lens.  It matters not which or what camera you use it on. A lens can not change its physical characteristics unless a mechanical modification is made. They are what they are.

[ebiggs1]

"... is a little soft at minimum aperture, but razor sharp at f/8.  But, I perform AFMA adjustments on it ..."

One caveat, AFMA does not make a lens any sharper. It simply moves the critical focus point closer or farther from the film plane. Again, back to that fact a lens is what it is and it always will be. Unless mechanical mods are done, of course, goes without saying.

[Robodot]

is that a f4 lens and are you using a crop sensor camera?

[Waddizzle]

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@Robodot wrote:

is that a f4 lens and are you using a crop sensor camera?

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I take it that you are still not convinced that aperture crop factor does not exist?

Okay,  Do not take our word for it.  Go see for yourself.  Buy a light meter and read the instruction manual.  Or, you could go visit a leading manufacturer’s web site, like Sekonic, and download an instruction manual.  It does not matter which model.  You can go for one of the digital models, but the inexpensive analog meters are simpler to use, and really drive the point home.

Exposure can be looked at as a way to describe light intensity striking an object, or a surface.  A light meter measures the intensity of light falling on its sensor, and can display the measurement as a camera exposure setting, or as an Exposure Value, or both.

Back to the instruction manual.  You do not really need to read it cover to cover.  I just want you to notice something abut light meters.  A light meter does not care what type of camera you have.  There is no provision to enter a crop factor for aperture.  Look for it in the manual, if you want.  You will not find it, no matter which manual you read.

The meter does not care if you have a full frame digital sensor, or an APS-C digital sensor.  A meter does not care if you are using a digital camera, a 35mm film camera, or a medium format film or digital camera.  All of those cameras would use the same exposure settings when given identical shooting conditions.

[Robodot]

here is the link to video from Tony explaining it better:
https://www.youtube.com/watch?v=hi_CkZ0sGAw

[Robodot]

I am sure all of you are right, I am just trying to figure out why my lens is only sharp at 300mm at f9. when i purchased it Canon says it is a f4/5.6 which i thought was the aperture settings I could use, thats not the case.

That really limits what I can do with it. I went by the f stop rating on the lens figuring at 5.6 I would get clear and sharp photos at 300mm.

So in looking for answers I ran across this video by Tony.

https://www.youtube.com/watch?v=hi_CkZ0sGAw

it just happens his formula of including the crop factor into the aperture rating too works out to be f9, which is where this lens is sharp.

[Robodot]

I like the word "Photons", has a nice ring to it.