Newbie question regarding mtf curves

[Joseph Thomas]

Hello,

I´ve a newbie question regarding the interpretation of mtf curves. - I
read a lot of websites about this stuff and learned many interesting
things but I found no explanations for the following.
Fuji states for Velvia 100 prof a resolving power of 80 lines/mm (I
guess they mean line pairs) at a chart contrast of 1,6:1 and 160
lines/mm at a chart contrast of 1000:1. Are those figures somehow
derived from the mtf curve they show on the same page of the specs and
if so, how is it done?
As far as lenses are concerned resolution is defined as the spatial
frequency at which mtf drops below some given value. 1% is a reasonable
value for a bright image and a high contrast bar pattern. - Is this
"given value" related to the threshold of visibility of our visual
system?

Can someone please shed some light on this for me?!
Joseph Thomas

[Peter]

Post by Joseph Thomas
Hello,
I´ve a newbie question regarding the interpretation of mtf curves. - I
read a lot of websites about this stuff and learned many interesting
things but I found no explanations for the following.
Fuji states for Velvia 100 prof a resolving power of 80 lines/mm (I
guess they mean line pairs) at a chart contrast of 1,6:1 and 160
lines/mm at a chart contrast of 1000:1. Are those figures somehow
derived from the mtf curve they show on the same page of the specs and
if so, how is it done?

There is no direct relationship between the two in practice even if
one would think there ought to be one.

Compare the MTF curve of Kodak Tmax 100 with Fuji Velvia 50.
The MTF curve of Tmax 100 is more than twice as good.
Velvia 50 has around 100% MTF at around 22 cycles/mm and
50% MTF at around 50 cycles/mm. Tmax 100 has 100%MTF
at around 50 cycles/mm and 50% MTF at around 120 cycles/mm.

Then compare the 1.6:1 contrast resolution figures. Velvia boasts
80 lp/mm while Tmax 100 shows only 63 lp/mm. This is
probably only a just noticable difference in the test, but
it does seem odd that Velvia is slightly better on the low contrast
resolution chart test while Tmax 100 shows greatly superior
MTF test results.

I don't know why this is the case, but it certainly means that you
can't necessarily predict the results of one test from the other.
It is generally claimed that the MTF curves provide the more
useful information.

As I understand things, the resolution chart tests are rather
old-fashioned. A test target with a contrast of 1.6:1 (two
somewhat different shades of grey) is photographed with
an extremely good lens and the negative or slide is examined
under a microscope to find out which sets of bars are just barely
distinguishable as bars.

The 1000:1 test target results are not meaningful for pictorial
photography, though they may be useful for astrophotography.
The 1.6:1 test target is a fairly strict test because the two shades
of grey are only about 2/3 of a stop different and must still
be distinguishable at the stated frequency.

Post by Joseph Thomas
As far as lenses are concerned resolution is defined as the spatial
frequency at which mtf drops below some given value. 1% is a reasonable
value for a bright image and a high contrast bar pattern. - Is this
"given value" related to the threshold of visibility of our visual
system?

Most of the time an MTF figure of 20% to 30% is regarded as
useful for pictorial photography. Note that this is for the whole
system of camera lens plus film plus enlarging lens. If you
multiply 60% x 60% x 60%, you get only 21.6%, so in general
you would want each element in the system to have an MTF
of at least 50% at the highest frequency you are interested in.

Peter.

[Joseph Thomas]

Thanks Peter for the fine explanation.
Now I know how those resolving values are derived from. I thought they
were somehow contained in the mtf curves.

Joseph Thomas

[Jim]

Post by Joseph Thomas
Thanks Peter for the fine explanation.
Now I know how those resolving values are derived from. I thought they
were somehow contained in the mtf curves.
Joseph Thomas

Aren't the resolution values determined by examining the image of the test
target?
Jim