Does the 90D have a real focus problem?

Started 2 months ago | Discussions thread
jrkliny
jrkliny Veteran Member • Posts: 4,238
Re: Does the 90D have a real focus problem?

David1961 wrote:

jrkliny wrote:

First shoot at a high enough shutter speed to avoid camera shake and motion. In the past that meant a shutter speed greater than the focal length. For example, when shooting at 50mm be sure to have a shutter speed faster than 1/50.

That is not correct in every case because that rule-of-thumb is based of FF eq. focal length.

If the 50mm lens is on a 1.6x crop sensor then the minimum shutter speed should be 1/(50x1.6)s.

I'm sure you can do the maths.

To be more precise, the rule of thumb is based on effective focal length.  If you have a 50mm EF-S lens the effective focal length is the same as the nominal and you should shoot at 1/50 for faster.  Of course that is an old rule based on older cameras with lower resolution.  I would recommend shooting substantially faster with the 90D and other higher resolution modern cameras.

If you put on a 50 mm EF lens, then the rule would mean that you should correct by 50x1.6 = 80.  80 is the effective range for this lens for a 1.6 APS-C sensor and the minimum shutter speed would be 1/80.

Many lenses also have up to 4 fold image stabilization.   For a 50mm effective you should theoretically be able to shoot at 1/12 second.  For that 50mm EF lens with an effective focal distance of 80, you should theoretically be able to shoot as fast as 1/20 s.

In any case your point and the theoretical considerations are making this way more complicated than needed.  The point is simply that you are likely to have a blurry image if you shoot at a slow shutter speed.  Personally I would not want to rely on the performance of image stabilization and I also try to shoot considerably faster than the old rule of thumb would allow.  Even that will not compensate for bad technique.  Nor does it take into account any movement of the subject.

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