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Basic Photo Tips: Aperture, Shutter Speed, and ISO

Just as it was 100 years ago and just as it is today, every camera—be it film or digital—is nothing more than a lightproof box with a lens at one end and light sensitive film or a digital...

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The September Monthly Project Read More

The September Monthly Project

This month's project with guest instructor Jackie DiBenedetto focuses on challenges - and joys! - of photographing kids. Add your best photo to the thread and enjoy the conversation!

What is TTL and non-TTL

Amy Perl , Oct 17, 2007; 01:04 a.m.

Ok, I am finally going to ask because i don't know. What is a TTL flash, versus a non-TTL flash?


Peter Lyons , Oct 17, 2007; 01:10 a.m.

TTL means "through the lens". Flash metering is through the lens, using the camera's metering system, which looks through the lens you're using. Non-TTL means the flash doesn't communicate with the camera in that way, and instead uses its own light metering system, built into the flash. TTL is better!

Peter Lyons , Oct 17, 2007; 01:15 a.m.

To elaborate... TTL is better because it evaluates the need for flash based on what you're actually looking at through the lens you've chosen, whether a 24mm or a 300mm, rather than based on a much more general light reading taken by the flash itself. TTL is susceptible to the same metering errors that any auto-exposure program is, but still offers an advantage over non-TTL. I remember, back in the good ol' days, buying a Vivitar flash for my Canon AE-1 Program. It was non-TTL, and had a little round sensor on the front of it for light metering. Today, my 580 EX communicates directly with my 5D very nicely. Except when the flash contacts are coated with salt. <sigh...>

Ilkka , Oct 17, 2007; 04:20 a.m.

It is a bit simplistic to just say that 'TTL is better', because all studio flashes are not TTL and are often quite obviously better sources of light than a small flash unit made by the camera manufacturer.

Colin Southern , Oct 17, 2007; 04:45 a.m.

Ilkka wrote: "It is a bit simplistic to just say that 'TTL is better', because all studio flashes are not TTL and are often quite obviously better sources of light than a small flash unit made by the camera manufacturer."

TTL is a flash metering technology for cameras - not camera lighting. Nothing to do with quality or source of the lighting.

Take a peak at http://photonotes.org/articles/eos-flash/#ttl if you're keen.



Sam Thompson , Oct 17, 2007; 05:46 a.m.

A TTL flash will take measurements from the camera, what aperture you are setting, shutter speed, and ISO and then compute a flash output. Modern digital cameras use a series of preflashes to determine exposure.

A non TTL automatic flash you tell your flash what aperture and ISO you will be using then the flash will compute a flash output based on how much light is available.

An adjustable manual flash, you set your flash output manually based on what aperture and iso settings you are using. You use a flash meter to determine what you should set the camera to or you use a complex set of mathematical formulas.

A non adjustable manual flash, you don't set any settings on the flash, it will flash at full power. You control your exposure by setting your camera to a certain ISO and aperture depending on how far the subject is.

Nathan Stiles , Oct 18, 2007; 12:50 p.m.

I googled "what is TTL flash", and you may want to check out the first link returned. Google is a fantastic tool for knowledge.


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