Shutter speed.
 

Over on the Bird forum I put some advice on a thread about minimising camera shake saying to use 1/focal length as a general guide, not forgetting to put in the crop factor so for a Canon with a 400mm lens that would mean 1/640 sec. Postcardcv came back and said you did not have to allow for the crop factor so 1/400 would be sufficient even though the lens is effectually a 640. Who is right? Has anyone got any authoritative references? :confused:

Mike,

Can't offer any authoritative references, but I always tell people to use the fastest they can get away with - it doesn't (or shouldn't!) take long to work out what you can successfully hand hold at. Sometimes it is not possible to get the shutter speed you want, so you just do your best.

For example take my current picture of the Jay in the gallery - I pushed the ISO up to 1600 but still only got 1/125 at 550mm, so I made sure I was rock steady and got away with it... guides should only be guides etc.

The short answer is that it depends.
Let's forget digital for the moment and just think about film as pixel densities just confuse the issue. To start with consider Peter's case. A given lens at a set subject distance will produce a given image height on the film regardless if the film was full frame 35mm (24mm height) or APC (15mm height). Assume the image height is 5mm and we want the image on the print to be 5cm the amount of enlargement will be x10. In this case the crop factor plays no part in the equation.

On the otherhand if the image was to fill the height of a full frame 35mm (24mm height) camera it would over fill the height of the APS-C (15mm height) frame because of the reduced field of view. Therefore using the same lens for a APS-C camera the same subject has to move further away to fit into the smaller field of view. This where the effective field of view of a 400mm lens becomes 640mm on a 1.6 crop factor camera. At this point Peter's statement is still true.

The problem starts when we come to printing the images.
The amount of enlargement required to produce an A4 print from the full frame 24mm high image is x8.75 compared to x14 for the APS-C 15mm high image. This is 60% extra enlargement which will show up camera shake problems more.

This extra enlargement also has an effect of Depth of Field Calculations and Bokeh. A reason why some prefer a full frame sensor to a APS-C sensor.

I use shutter speed this way, If my focal length is say 45mm then I use a shutter speed of 1/100s or as near as I can get it. If the aperture does not give you enough light then alter your ISO until it looks right. You need twice the shutter speed for the focal length you are using. A word of caution. Even though you can sometimes get away with it, if you use a slow shutter speed then use a tripod.

I am not sure that the focal length rule works anyway on long lenses. I can handhold my 400/5.6 at, say, 1/500 and get sharp shots most times but if I add a 1.4tc bringing it up to 560mm I struggle to get sharp shots at 1/1000 or even faster (when using a tc I always use a tripod which gives me good results).

I have a theory that the focal length=shutter speed rule works on short focal lengths (say 100mm or less) but as you go up the focal length it becomes less effective. Of course IS comes into it as well.

I do not know if you should add the crop factor or not but to be on the safe side it is worth while to add it.

I just posted my thinking up on BF but thought I'd put it here too -

If I take a shot with a 1DsmkII it's full frame at 16.7mp, if I then crop the image to 1.3x then I get left with ~9.8mp (mich is roughly what a 1DmkIII gives) but if cropped to 1.6x I'd be left with ~6.5mp (basically what the old 300D gave). If I took the same shot with the same lens with each of these three bodies then I would need the same shutter speed to eliminate camera shake. The central portion of the three images should be the same, the larger sensors purely record more data around the edges. So by my thinkng crop factor cannot have any effect on the shutter speed you need to eliminate camera shake. (I'm not sure that I made my thinkng any clearer with that).

The focal length=shutter speed rule can only ever be a guide, personally I can get away with slower shutter speeds. When I had then 400 f5.6 I was happy to handhold down to 1/200th and at times slower.

I'm in agreement with Rob, it all comes down to the printing. If you don't take into account the crop factor increase, you are doing a bigger enlargement from sensor to print, at which point the increased amount of shake will become apparent.

Another way of looking at the problem.

In this example let us assume the camera shake causes a 1mm vertical movement of image on the sensor which will be true for both full frame and APS-C sensor. In the case of full frame this movement equates to 4.1% of the frame and 6.6% for APS-C. It is only when you use different scaling to make the same size print from each system the shake becomes more apparent.

If you use the same size area of the sensor in each case and apply the same amount of scaling the shake will appear the same.

Remember 1/focal length Sec is only a general rule of thumb (guide) which works for most people.

I am of the opinion that as you get older you are less able to hold the camera still at low shutter speeds. I find the IS on my 300 F4 a boom for hand held shots but not as good as tripod.

Thanks for your clear expositions Rob, I think I now understand it. Although it is only a rule of thumb which changes from person to person and indeed, I agree, changes over one's lifetime it is a useful guide when out and about and looking at the camera settings and deciding what alterations are needed.

I get what you're saying but either way there's still camera shake and it will be visable on the image - if you start looking at it in this way then surely pixel pitch and thus mp count become relevant too. Surely a shutter speed that is fast enough to stop camera shake on a full frame senser will still eliminate shake when using a crop body. I have used full frame, 1.3x crop and 1.6x crop cameras and can handhold at the same shutter speeds on all three. For me any camera shake is too much, if an image isn't critically sharp then it gets deleted.