Of course if the shorter FL lens is focused at its HFD and the longer one isn't, the difference in DOF will be even greater since the back DOF at HFD is infinity.

By use of a laser diode as a pump source, a self-Q-switched laser from a Cr, Nd:YAG crystal is demonstrated. The output Q-switched traces are very stable, the threshold pump power is 3.5 W, the pulse duration is 50 ns, and the slope efficiency is as high as 20%. In addition, the pulse width remains constant while the pulse repetition rate varies with pump power.

So aperture is one thing to control and the second thing to control is how blurred u want your backgrounds to be. To get the best control, keep your backgrounds out of the DOF area and then select the focal length that will seem to reduce or increase the sharpness of the background area to your liking.

The statement that DOF doesn't depend on focal length but only on magnification and aperture is correct except when the focusing distance for one of the focal lengths is near the hyperfocal distance and the other one isn't. If both are focused at 1/8 the hyperfocal distance or closer, then the DOFs are essentially equal (for the same mag & f-number). If one is focused at 1/4 HFD or further then the differences start to become significant.

Thanks everybody for your comments. I think I finally understand why telephoto lenses SEEM to shorten DOF. It's really just that you are inclucing less background.

By using a continuous-wave Ti:sapphire laser as a pumping source, we demonstrated a passively Q-switched Yb:YAG laser at room temperature with Cr(4+):YAG as the saturable absorber. We achieved an average output power of as much as 55 mW at 1.03 mum with a pulse width (FWHM) as short as 350 ns. The initial transmission of the Cr(4+):YAG has an effect on the pulse duration (FWHM) and the repetition rate of the Yb:YAG passively Q-switched laser. The Yb:YAG crystal can be a most promising passively Q-switched laser crystal for compact, efficient, solid-state lasers.

The only real difference you'll see is from the slightly different background.A longer lens for example with have a very narrow view of the background while a wide lens sees alot more ,even though the subject is framed identically

We have developed all-solid-state continuous-wave diode end-pumped Nd:YAG and Nd:GdVO4 lasers that were passively Q-switched by Cr4+:YAG saturable absorbers. The Nd:YAG laser delivered laser pulses with 138 mJ maximum energy and duration of 42 ns, with 3.3 W average power at 1.06 mm for 13.3 W of pump power at 807 nm. An average power at 1.06 mm of 1.4 W was obtained from Nd:GdVO4 under pumping with 7.2 W power at 808 nm. The pulse energy and the pulse duration were 41 mJ and 40 ns, respectively, which correspond to 1 kW pulse peak power. The laser pulses characteristics are discussed for Cr4+:YAG saturable absorbers with different values of the initial transmission.

Conversely then, wouldn't a telephoto lens actually include more foreground objects, make them appear smaller, and therefore in better focus?

Also if the background is outside of the DOF field of the lens, the blur *SEEMS* more prominent in a longer lens because the same blurred takes up more print area when using a longer lens. This is because by definition, when the subject framing is exactly the same the background covered by the lens will be larger for wider lenses giving an illusion of more sharpness.

I found some information on the internet about how to calculate your depth of field given your format (35mm or other), lens length, focal length, and aperture. It assumes print sizes of 8x10". It also assumes a single element lens, but is supposed to be pretty accurate for most lenses at a moderate focusing distance.

James P. - That's a good article. I especially like how he enlarged the image of the building from the wide shot to show you that its focus was the same as in the telephoto shot.

The results in research and development of diode-pumped passively Q-switched Nd:YVO4 lasers are presented with Cr4+:YAG crystal used as an intra-cavity saturable absorber. Using a-cut Nd:YVO4 laser crystals, the passively Q-switched lasers have been successfully developed and provided stable single shortest pulses of 40 ns at the repetition rate of 18.5 kHz, the maximal average output power of 180 mW was achieved corresponding to the optical efficiency of 9% and the slope efficiency of 16%. The dependences of average output power, repetition rate, pulse width and peak power on pump diode power for different transmissions of output mirror are also investigated.

For example 50 and 100mm lenses focused at a distance to give a magnification of 0.012 (or imaging a 5 ft high subject on 18mm of film). At f/16 the 50mm lens has a total DOF (front to back) of about 66 feet, and the 100mm lens has a total DOF of only about 27 ft. This would be significant for a landscape scene.

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Rather what most people worry is the Bokeh of the background. I wont go into what is better, but just to give an explanation, the out of focus shape of a small point of light area in the background is different for different lenses.

BUT...Once you start using the same framing (same magnification)for different lenses,then DOF will be identical at any given aperture

Using this information, I made a spreadsheet to verify my test results. At the closest focusing distance and widest angle of view (@28mm), my DOF is calculated to be about 600mm. At my farthest focusing distance and narrowest angle of view (@75mm), my DOF is calculated to be just over 500mm. Also, beyond the 75mm focal lenght point, there is almost no change in DOF. So, interestingly, at least according to these simplified calculations, DOF actually does change, but perhaps not enough to make that much of a difference (especially since 75mm is the MINIMUM focal length a portraint photographer would consider). It appears that the best way to affect DOF is to change your aperture size.

Depth of field doesnt change significantly after 70mm. When u go from a short lens to a longer lens but keep the framing same, The DOF decrease due to the increased focal length tracks nearly with the DOF increase due to the further distance from the subject. Unless u are doing real shallow DOF photos with 50 mm or 35 mm lens you should not worry about it.