How to calculate tube length?

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len
Posts: 66
Joined: Sat Dec 26, 2009 5:51 am

How to calculate tube length?

Post by len »

Hi,
I'm experimenting with 4x objective and would like to know if I calculate the tube length correctly.

I have a T2 Screw Thread for Canon EOS EF connected to M42 Converter Cone.
The T2 screw has a 5mm male screw which inserted completely into EOS mount so I exclude it from the total length.
EOS EF camera has FFD (flange focal distance) of 44mm.

So does the total tube length equivalent to : 44 + 64 = 108mm
Does it mean that if I want to use optimal tube length I need to add 150 - 108 = 42mm extension ring?
Attachments
length2.jpg

Pau
Site Admin
Posts: 5560
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Re: How to calculate tube length?

Post by Pau »

The tube length for direct projection with finite microscope objectives is measured from the objective shoulder to the sensor plane. If the objective is marked 160 this distance is 150mm as you say and in this case it will deliver the nominal 4X magnification.
Your measurement and calculation seem right.
Low NA (<0.35) objectives like yours are pretty tolerant to changes in the TL, if shorter the magnification will be smaller, the image circle smaller and sharpness at the centre can be higher at cost of degraded periphery. If longer, the reverse applies.
Pau

len
Posts: 66
Joined: Sat Dec 26, 2009 5:51 am

Re: How to calculate tube length?

Post by len »

Pau wrote:
Fri Dec 10, 2021 4:11 am
The tube length for direct projection with finite microscope objectives is measured from the objective shoulder to the sensor plane. If the objective is marked 160 this distance is 150mm as you say and in this case it will deliver the nominal 4X magnification.
Your measurement and calculation seem right.
Low NA (<0.35) objectives like yours are pretty tolerant to changes in the TL, if shorter the magnification will be smaller, the image circle smaller and sharpness at the centre can be higher at cost of degraded periphery. If longer, the reverse applies.
Thanks Pau.

CharlesT
Posts: 8
Joined: Sat Feb 01, 2020 9:02 am

Re: How to calculate tube length?

Post by CharlesT »

Be aware that the black coating on inside of some of those cone adapters can be very reflective; to get reasonable photographs it might need to be lined with something matt black ; try some black paper or thinnish black EVA craft foam and see if that helps at all.
Canon 600d
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