Avoiding blur: shutter speed, focal & distance to subject

[rhaytana__tim_adams_]

I'd like more information on the relationship between focal length and

distance to subject and the minimum shutter speed that can be used

with flash.

 

A picture is worth a thousand words; please see the included photo.

 

I have used a Nikon F100 with an SB-80 flash and an autofocus 28-70

f/2.8 ED-IF lens for several years now. I do many indoor close-ups,

three to six feet from the subject, with the SB-80 flash set to

Nikon's 3D Matrix "fill flash" mode. With this setting, I routinely

drag the shutter down to 1/25, 1/20 or even 1/15, and have been quite

happy with the results. The lens is a 28-70, but it's usually at 28 -

40 in these types of shots. Blur is sometimes a problem at 1/15, but

usually not.

 

A few weeks ago, I used these camera settings when farther away from

the subject -- I'd guess about 12 to 15 feet -- and with the lens at

between 50 and 70. The badly blurred results, shown below, were an

unpleasant surprise for me, and a wake up call that I need to go back

and learn something I should have learned a long time ago:

 

There's obviously a relationship between distance to subject and the

minimum shutter speed I can use with flash, while avoiding blur. If a

more experienced photographer can fill me in, I'll be grateful. I'm

guessing that I need to go to higher shutter speeds when the subject

is farther away from me.

 

(Of course, this assumes that the subject is standing still -- and the

fellow pictured was standing still most of the time, although he had a

knack for moving just as my finger went down on the shutter button.)

[cappoldt]

Tim, my guess is that those images you took at a closer range were with the lens at or near the 28mm focal length, allowing an acceptably sharp image due to the relationship of focal length and shutter speed (you don't want to drop below the inverse of the focal length in shutter- so 28mm requires a minimum shutter of 1/28, or on your camera, likely 1/30). Also, that's pretty lucky - few can hold a camera still enough at that shutter speed.

 

From further away, at 50-70mm, you need a minimum shutter of 1/50-1/80 sec. . .and the added focal length also multiplies the blur of camera shake.

 

The flash, on matrix fill, is only doing what you tell it to. . .filling in at the given exposure settings...NOT compensating for your slow shutter, which is what's causing the blur. . .you just can't expect to keep the hand-held camera still enough below 1/60 sec.

 

What I think is that you're correct - you want to increase your shutter speed to about 1/100 minimum, adjusting your film selection's ISO to a higher number (try ISO 800) and keeping your f-stop wide open. That way, the matrix fill flash will pump enough light to compensate, and you'll get sharp pics.

[iwong]

With flash, you can steal a couple of stops of shutter speed from the 1/focal length rule because the flash will freeze motion of your subject. Dragging the shutter will help bring in more ambient light to lessen the "deer in a headlight" look, but too weak a flash, or too small an aperture opening, will cause the flash to illuminate longer, which negates the motion freezing effect.

 

Unless you need the DOF of f/11, I would open up the aperture more to help the flash, and set the shutter at 1/30 or 1/60, depending on ambient light levels. Unless the venue is very dark, I wouldn't normally risk 1/8 or 1/15 unless I use a 28mm lens and no more. But then I still have to brace my elbows or body on something solid to help stability.

 

Hope this helps.

[neal_shields]

Anytime that you take a picture with a flash you are balanceing the light from the flash against your ambiant light.

 

If the light from the flash is several stops brighter than the ambiant light then your flash duration is what is stopping the action and your shutter speed doesn't make any difference. As you get farthar away from the subject the ambiant light tends to overpower your flash and then your shutter speed is what stops the action or camera vibration.

 

Normally the rule of thumb is your shutter speed must be at least as great as your focal lingth for hand holding. I.E. 50 mm lens don't go below 1/50th of a second.

 

Normally most shoe mount electronic flashes have a flash duration of 1/1000 of a second. However as you get closer to your subject the flash automatically reduces power to keep from over exposeing. This is done by shorting the flash duration. When you get down to about 1/16 power your flash duration can be down to about 1/15000 of a second which is sufficent to stop a speeding bullet.

 

I just did some experiments with a spinning disk and a shoe mount flash. I spun a 16" disk at 30,000 rpms and photographed it in total darkness. I was able to resolve a test target at the edge of the disk even though it was moving at 2000 ft. per second.

 

http://sueandneal.com/index.exe

[neal_shields]

Sorry wrong link:

 

http://truckgenerator.com/subdomain/sueandneal/strobe_speed.htm

[rhaytana__tim_adams_]

Christopher, Isaac and Neal, thanks very much for your responses. I feel lucky to be able to share with other photo.net users -- resolving issues like these would be far more difficult without access to such a helpful forum.

 

I have on disk -- believe it or not! -- a database of most of the 12 K images I've taken with the F100, complete with Photo Secretary info for each shot. I just searched through the database for all images taken at a focal of 60 and greater, shot indoors.

 

Sure enough, more of those images are blurred than those snapped with the lens between 28 - 40 focal length. But _some_ are quite sharp, even at 1/15 or 1/20. However, all the sharp photos at a higher focal length were snapped very close to the subject.

 

(I do usually make a special effort to hold the camera still when shooting at slow shutter speeds: holding my breath, bracing elbow against my chest, bracing body against wall or elbow on support, etc.)

 

Neal, what you wrote sounds especially relevant here: that the flash is freezing the action close-up, but that the shutter speed is what stops the action when I'm farther away.

 

Time for some experimentation. Once again, thank you for taking the time to fill me in!

 

I'll ask a secondary question, as I may soon upgrade to digital equipment: does the shutter speed => focal length rule change when dealing with a crop factor of 1.3 or 1.5, typical of digital equipment? I'd guess that the 17 mm lens becomes a 25 mm lens, and that I should then keep the shutter speed above 1/25. But, I don't have any practical experience here.

[Jerry_]

If you want to experiment, try using the camera's large spot meter, put the camera in 'S' mode at 1/100th second. Then dial in +.3 or +.5 on the exposure compensation. Now try shooting a few frames at 70mm__the blur should be gone. You can even take a few shots at 1/250th for the sake of experimenting. (It works for indoor action like basketball, which is a bit faster than the guy on stage.)

 

 

 

The 'matrix' meter is very good but indoor flash work sometimes improves with a spot meter selected on the F100.

[gaius1]

The Kodak Professional Photoguide has a little table of different types of movement and that shutter speed needed to freeze them.

[steve_muntz]

I guess from the perspective of shooting something like that, I'd probably use f4 or f5.6. That would help with the shutter speed, and you don't need any more depth of field (in this case) than that. Others are right about the rule of focal length to shutter speed, and using an aperture like 4 or 5.6 would get you a shutter speed more suitable for this.

 

As far as the digital conversion with a 1.5 sensor, play it safe and use the "equivalent" focal length when figuring this (example, 50mm lens counts as 75mm lens, use 1/75 sec). In reality I find that I don't really need this much of a difference, but maybe somewhere between the two (1 and 1.5). As long as I lay off the coffee, anyway.

[rhaytana__tim_adams_]

Gerald, Guy and Steve, thanks for the additional feedback. I'll keep your tips in mind next time I'm out with the camera.